Dr. Richard Green, one of the earliest and most vocal critics of psychiatry’s classification of homosexuality as a mental disorder, died on April 6 at his home in London. He was 82.

The cause was esophageal cancer, his son, Adam Hines-Green, said.

Dr. Green, who was also a forceful advocate for gay and transgender rights in a series of landmark discrimination trials, became aware of the marginalization of people because of their sexual and gender identities while training to be a doctor at Johns Hopkins University in Baltimore, a leader in the study of sexuality.

In 1972, shortly after completing his specialty in psychiatry, he defied the advice of colleagues and wrote a paper in The International Journal of Psychiatry questioning “the premise that homosexuality is a disease or a homosexual is inferior.”

At the time, three years after the protests against a police raid at the Stonewall Inn in New York, a turning point in the gay rights movement, psychiatry’s diagnostic manual classified homosexuality as a mental disorder, and publicly arguing otherwise came with professional risks.