By GIOIA DILIBERTO

Farewell to Arms,'' Ernest Hemingway's masterpiece about a doomed World War I romance, is one of American literature's most enduring classics. Since it was first published in 1929, it has sold steadily, even during the 60's, after the author's death, when his literary fascination with violence caused his other works' popularity to drop.

Scholars have long known that the novel was based in part on Hemingway's romance with Agnes von Kurowsky, a pretty, coquettish American nurse whom he met after being wounded on the Italian front while working as a volunteer for the American Red Cross. At 26, von Kurowsky was seven years older than Hemingway. Still, Hemingway had hoped to marry her. He was devastated when she rejected him after the war, addressing him as ''kid'' in a Dear John letter and saying she was fond of him ''more as a mother than a sweetheart.''

The effect of the relationship on Hemingway's life and career has intrigued scholars for years. The details of the wartime romance, however, remain obscured, primarily by the differing accounts of the two lovers. In later years, von Kurowsky insisted it had been an innocent flirtation, while Hemingway maintained they'd had a sexual affair.

Now a new movie, ''In Love and War,'' which opened on Friday, claims to reveal the true story. This $40 million soap opera is not likely to resolve the mystery, but it does prove that the Hemingway myth is alive and well. Directed by Richard Attenborough and starring Chris O'Donnell as Hemingway and Sandra Bullock as von Kurowsky, it's based on the nurse's World War I diary and letters, extraordinary documents that came into the film maker's hands by a turn of fate.

The fact that the movie got made at all is due to the producer Dimitri Villard's father, Henry Serrano Villard, who befriended Hemingway and von Kurowsky during World War I. Like Hemingway, Villard was a volunteer for the Red Cross ambulance service and he was treated for hepatitis at the Milan hospital where von Kurowsky was posted. At the time, his father was unaware of ''Ag and Ernie's'' attachment, says Dimitri Villard, though ''he once saw them holding hands'' in a way that suggested ''Agnes wasn't taking Hemingway's pulse.''

Villard lost touch with the couple after the war. Then, in 1961, soon after Hemingway's suicide, Carlos Baker, the writer's biographer, contacted Villard and gave him von Kurowsky's address. He corresponded with the former nurse, who was now married and living in Key West, Fla., where Hemingway also had lived.

Later, at von Kurowsky's urging, Villard used his influence as a former American ambassador (to Libya, Senegal and Mauritania) to win permission for her to buried at the National Cemetery in Arlington, Va. After her death at 92 in 1984, her husband, in gratitude, sent her diary and letters to Villard ''out of the blue one day,'' says his son.

Henry S. Villard (who died last year) and James Nagel, a Hemingway expert who teaches literature at the University of Georgia, published the material in a 1989 book, ''Hemingway in Love and War.'' In a pain stakingly-researched essay, Nagel wrote that von Kurowsky returned home a virgin. But in the film, the couple consummate their affair at a flophouse on the Italian front. As they undress and fall into an embrace, a dreamy Italian melody wafts through the open window mixed with chatter of prostitutes. ''Oh, Aggie, it wasn't supposed to be like this. We were going to be in the most beautiful place on God's earth,'' says Mr. O'Donnell's character. ''Never mind . . . close your eyes,'' his nurse whispers back.

This sentimental scene has no basis in fact, though Dimitri Villard says it's in the ''spirit'' of the truth; he is convinced that the couple consummated their love. ''Agnes's letters are so passionate it's hard to believe there wasn't a relationship between the two,'' he says.

''Kid, my kid,'' von Kurowsky wrote Hemingway on Sept. 25, 1918. ''I've just been in your room, and talk about chairs that whisper! That whole room haunted me so that I could not stay in it.'' And on Oct. 17, she wrote, ''I guess every girl likes to have some man tell her . . . he can't do without her. Anyway, I am but human, and when you say these things I love it and can't help but believe you so don't be afraid I'll get tired of you. I haven't really started to worry yet over your forgetting to love me as you do now.''

But these musings -- written after the two were separated first by a trip he took to Stresa, then by her transfer to a hospital in Florence -- are in sharp contrast to the tart entries in her diary written not long before.

''Now Ernest Hemingway has a crush on me or thinks he has,'' she wrote on Aug. 25. And in other typical passages, she wrote, ''Ernest Hemingway . . . is far too fond of me and speaks in such a desperate way every time I am cool, that I dare not damper his ardor as long as he is here in the hospital. Poor kid, I am sorry for him.''

Still, the idea of a torrid affair between the teen-ager from Oak Park, Ill., and the shapely auburn-haired nurse, fits the myth of Hemingway as an icon of male prowess -- hunter, drinker, fighter, writer and lover. Perhaps that's why the myth of the affair persists in the mostly male world of Hemingway scholarship, and why it has led to some outlandish speculation. In his 1987 biography, ''Hemingway,'' Kenneth S. Lynn, then a professor at Johns Hopkins University, described in detail the couple's lovemaking, even the position they preferred.

The known facts, however, suggest that the romance was entirely innocent. Indeed, the only evidence of their possible intimacy was von Kurowsky's hairpin, which was found by another nurse under Hemingway's pillow. And, of course, the writer's own boasting in letters to friends. But, as Mr. Nagel points out about Hemingway's correspondence, ''there's a long series of letters during this period that we know to be phony,'' in the sense that Hemingway bragged or embellished the truth. One letter, for instance, announced his engagement to Mae Marsh, the once-celebrated but now-forgotten silent film star.

Von Kurowsky herself denied having slept with Hemingway. ''Let's get it straight -- please,'' she told Henry S. Villard, ''I wasn't that kind of girl.''

But in fact what happened is less important than the emotional aftermath of the experience for Hemingway -- the effect it had on his relationships with women and the portrayal of women in his fiction.

The film's love scenes may not be historically accurate, but they capture what Mr. Attenborough says is a mood of emotional intensity, and this is ''the heart of the matter.''

''Do you think if they'd gotten married and had children that he would have been the same writer?'' Mr. Attenborough asks. ''Do you think the anger and frustration and humiliation he suffered in his belly for so many years in regard to being turned down, which drove him and fired him, would still be there?''

Perhaps not. It's clear Hemingway continued throughout his life to draw on von Kurowsky in creating his heroines. The book von Kurowsky is best known for inspiring is ''A Farewell to Arms.'' As anyone who has taken a college literature course (or seen the 1932 movie starring Gary Cooper and Helen Hayes, or the less interesting 1957 movie with Rock Hudson and Jennifer Jones) knows, it tells the tragic love story of Catherine Barkley, an alluring British nurse, and Frederic Henry, an American lieutenant in the Italian Army, who deserts his unit. The war setting and the lovers' occupations draw on Hemingway's experience but there are vast differences between real life and the novel. Among other things, Catherine dies after giving birth to a stillborn son.

Before making the movie, Mr. Attenborough asked Mr. O'Donnell and Ms. Bullock not to read ''A Farewell to Arms'' because he didn't want their portrayals to be influenced by Hemingway's fictional characters. Von Kurowsky's diary reveals her to be stalwart, somewhat insensitive and emotionally reserved. In contrast, Catherine Barkley, like most of Hemingway's heroines, is romantic and wounded, damaged by life in some fundamental way and desperate to escape her unhappy past through love. In this sense, she more closely resembles Hemingway's first two wives, the sensitive Hadley Richardson and the passionate Pauline Pfeiffer. (He wrote the book while married to Pfeiffer.)

Still, von Kurowsky was Hemingway's first love and her rejection triggered his literary preoccupation with romantic loss. In book after book and story after story, lovers are separated by misunderstandings, cooled passions, insanity and death.

Or, as in ''In Love and War,'' they're parted forever by pride. ''There's still a part of me that wants to take you in my arms . . . but I've changed . . . it wouldn't work,'' Hemingway tells von Kurowsky when she tracks him down at his parents' cottage in Michigan at the end of Mr. Attenborough's movie. In truth, the couple never saw each other after they left Italy in 1918.

''In a movie you have to make compromises,'' says Mr. Nagel, who was a consultant on the screenplay. ''When they called me and said, 'We think we have to bring them together again,' it made sense to me. It's unsatisfactory for them not to see each other again, for the romance to simply fritter away.''

Gioia Diliberto is the author of ''Hadley,'' a biography of Hadley Richardson, Ernest Hemingway's first wife.

At least, he adds, ''they didn't make it a happy ending.''

Agnes Stanfield