So the psychiatrist placed an ad in The Village Voice, inviting bisexuals to meet and discuss their experiences.

The weekly meetings grew into the Bisexual Forum, and provided Dr. Klein with material to develop the Klein Sexual Orientation Grid – which expanded on the zero-to-six Kinsey Scale.

Dr. Klein's pioneering research concluded that sexual orientation was too complex and fluid to be broken down into simple categories.

“He just had a tremendous vision,” said Regina Reinhardt, a San Diego psychologist and colleague of Dr. Klein's. “He just knew what needed to be done to educate the public. There isn't just a gay side and a heterosexual side. There are millions and millions of people who are bisexual. He felt that wasn't being acknowledged.”

Dr. Klein, a leader in the bisexual movement who was known worldwide, died May 24 at age 73. Though recently diagnosed with pancreatic cancer, he died from a heart attack at his San Diego home.

“He really, really embraced the essence of living the moment, in terms of not hanging onto the past and being fearful of the future,” said friend Carlos Legaspy, who served with Dr. Klein in the board of the Diversionary Theater in University Heights.

“He was very much at peace and happy with what life had given him and that was a true inspiration, an amazing inspiration,” Legaspy said.

Dr. Klein was born in Vienna, Austria, in 1932 and named Fred (he later changed his name to Fritz). When he was a boy, his family left Austria to escape anti-Semitism and the coming war, and made a new home in Brooklyn, N.Y.

The family eventually moved to Manhattan, where they operated a candy business.

Dr. Klein earned a medical degree at Bern University in Switzerland and an master's in business administration at Columbia University in New York.

Early in his career, he saw how little was known about sexual orientation, especially bisexuality. He created the Bisexual Forum, and in 1978 he published The Bisexual Option, which included in it the Klein Grid. “He took a system that Kinsey had started and put it in a 3D perspective. And as far as I know there's nothing that has surpassed it since,” said Dr. Klein's partner, Tom Reise of San Diego.

“It was the culmination of talking to hundreds of bisexuals in the Bisexual Forum in New York where the question of sexual orientation and bisexuality kept coming up,” Dr. Klein said recently in an interview with bimagazine.com. “It became obvious after a while that sexual orientation is more complicated than Kinsey's Scale.”

The Kinsey Scale measures sexual orientation, with zero being exclusively heterosexual and six being exclusively homosexual. It shows a continuum of sexual preference; Kinsey said nature rarely deals with discrete categories.

The Klein Grid expanded on the Kinsey Scale, taking into consideration a person's actual sexual experiences, as well as attractions, fantasies, emotional and social preferences, lifestyle and self-identification in the past, present and ideal future.

Reinhardt said, “For the first time people were able to recognize themselves in this. And it helps them deal with the uncertainties of their sexual orientation.”

In 1978, Dr. Klein co-wrote “Man, His Body, His Sex.” Four years later, after moving to San Diego, Dr. Klein founded the Bisexual Forum.

In San Diego, he continued his psychiatry practice and wrote several books, including a novel published last year called “Life, Sex and the Pursuit of Happiness.”

In 1998, Dr. Klein founded the American Institute of Bisexuality, a group that encourages research into and education about bisexuality. He served as the institute's chairman of the board.

Three years later, he founded The Journal of Bisexuality, a professional quarterly. He was its editor.

After his retirement from practice in the late 1990s, he continued lecturing at conferences around the world. When he wasn't lecturing or writing, Dr. Klein was promoting theater and the arts.

Legaspy said Dr. Klein believed in “the transformative power of art.”

“Being involved in the LGBT community, it's always a struggle to convey what it's like and the things you experience to people who don't have first hand interaction with LGBT folks,” Legaspy said of the lesbian, gay, bisexual and transgender community.

At the theater, the audience could share those experiences. And in doing so, minds could be opened, and stereotypes destroyed, Legaspy said.

In addition to Reise, Dr. Klein is survived by his brothers, George Klein and Seymour Klein of New York City. Dr. Klein donated his body to science.

Honoring his wishes that there be no mourning upon his death, his friends are planning a celebration of Dr. Klein's life. A date has not been set.