Jill Clayburgh, a two-time Academy Award nominee for best actress, who was best known for her realistic, emotionally searing portrayals in the late-1970s films "An Unmarried Woman" and "Starting Over," died Nov. 5 at her home in Lakeville, Conn. She was 66 and had battled leukemia for 21 years.

Ms. Clayburgh was part of the generation of female actors, including Sally Field, Faye Dunaway and Marsha Mason, who came of age during the feminist revolution of the 1960s. She brought an unflinching honesty to her roles, whether playing a prostitute in the 1975 television movie "Hustling," a Supreme Court justice in "First Monday in October" (1981) or a valium addict in "I'm Dancing as Fast as I Can" (1982).

Her signature role, however, came in 1978 when she played Erica, a jilted wife and mother in "An Unmarried Woman," directed by Paul Mazursky.

When casting the film, Mazursky said, "I was looking for three qualities: vulnerability, intelligence and a sexuality that wasn't brazen."

After her husband leaves her, Ms. Clayburgh's character has to pick up the pieces of her life. She becomes involved with other men and tries to forge ahead without losing her dignity.

The role was considered a breakthrough in screen acting for women, combining anger, vulnerability, overt sexuality - there were several nude scenes - and psychological depth.

Critic John Simon wrote in the National Review that Ms. Clayburgh portrayed "a woman rendered in all the complex interplay of antithetical impulses."

In 1988, entertainment critic Ryan Murphy wrote that Ms. Clayburgh's performance was "arguably the most well-rounded female character of '70s cinema."

Ms. Clayburgh won the best actress award at the Cannes Film Festival and was a favorite for the Academy Award but lost to Jane Fonda in "Coming Home."

In the 1979 romantic comedy "Starting Over," Ms. Clayburgh played a woman who becomes involved with a divorced man (Burt Reynolds) who can't get over his ex-wife (Candice Bergen).

Ms. Clayburgh received her second Oscar nomination for best actress but lost to Sally Field in "Norma Rae" - a role Ms. Clayburgh had turned down.

"Everyone couldn't understand why I turned it down," she told the Hartford Courant in 1999. "People thought I was nuts. And I was."

Ms. Clayburgh was born April 30, 1944, in New York City and grew up on Manhattan's Upper East Side. Her father was a well-off business executive and her mother was a secretary to Broadway producer David Merrick. Her grandmother had been an opera singer.

Ms. Clayburgh attended exclusive girls' schools and had her first taste of acting in the seventh grade when she played John Adams in a school play.

Soon after graduating from Sarah Lawrence College in Yonkers, N.Y., she appeared in her first movie, "The Wedding Party" (1969), an independent project directed by Brian DePalma and co-starring Robert DeNiro.

Ms. Clayburgh had several Broadway parts, including a starring role in the 1970 Jerry Bock-Sheldon Harnick musical "The Rothschilds," and appeared on the soap opera "Search for Tomorrow" for a year.

She had a small part in the 1972 screen adaptation of Philip Roth's "Portnoy's Complaint" and, the same year, won praise for her singing and acting on Broadway in the Bob Fosse production of "Pippin."

She returned to Broadway in 1974 in Tom Stoppard's "Jumpers," which premiered at Washington's Kennedy Center. The play was so unfinished in Washington that Ms. Clayburgh had to exit one scene by ripping a door off its hinges.

In the late 1960s and early 1970s, Ms. Clayburgh lived with actor Al Pacino for five years until, as she put it, he walked out on her.

She said she drew on her experiences in her breakthrough role in the 1975 ABC television movie "Hustling," in which she played a New York prostitute. She received an Emmy Award nomination for her role.

"I played this whore as funny, sensitive, childlike, and quite mad," she told American Film magazine, "a girl who just doesn't know how to make it, a state I know something about."

In 1976, she played opposite Gene Wilder in the film comedy "Silver Streak," and a year later starred in the football movie "Semi-Tough," as the team owner's daughter in love with two players.

After the early 1980s, Ms. Clayburgh stepped away from film to raise her children.

Survivors include her husband of 31 years, playwright David Rabe; two children; a stepson; and a brother.

Aside from occasional Broadway appearances, Ms. Clayburgh didn't resume her career in earnest until the late 1990s, when she had a recurring role as the mother of Calista Flockhart's title character on "Ally McBeal."

In 2005, she appeared in a revival of "Barefoot in the Park" on Broadway and, the same way, acted in a stage comedy, "A Naked Girl on the Appian Way," with her daughter, Lily Rabe.

After a long layoff from films, she returned in the 2002 romantic comedy "Never Again" with Jeffrey Tambor. In "Love and Other Drugs," a movie opening this month, she plays Jake Gyllenhaal's mother.

Ms. Clayburgh was proud of her understated acting style, believing it was more true to her characters and to life itself.

"I don't like being knocked out by performances," she told The Washington Post in 1978. "I don't want anyone to say, 'what a fantastic actor' about me."