Dr. Richard A. Isay, a psychiatrist, psychoanalyst and gay-rights advocate who did not admit to himself that he was gay until he was 40, married and a father, and who won a pitched battle to persuade his own profession to stop treating homosexuality as a disease, died on Thursday in Manhattan. He was 77.

The cause was cancer, said his son, David, the founder of StoryCorps, an oral-history project.

At his death, Dr. Isay (pronounced EYE-say) was a professor of psychiatry at Weill Cornell Medical College and a faculty member at the Columbia University Center for Psychoanalytic Training and Research.

“He changed the way the psychoanalytic world viewed the subject of homosexuality,” said Dr. Jack Drescher, a training and supervising analyst at the William Alanson White Institute in New York and the author of “Psychoanalytic Therapy and the Gay Man.” “He was a pioneer, a very brave man. He was attacked by psychoanalysts. He took a lot of flak.”

During the era in which Dr. Isay trained, homosexuality was viewed as a “lower level of psychological development,” Dr. Drescher said. It was something to be cured in therapy, and openly gay professionals were barred from training as analysts at institutions accredited by the American Psychoanalytic Association, the oldest professional group for analysts in the United States and one of the most influential, with many training institutes and affiliated societies.