Gay men “were literally dying to get into these trials,” said Jeff Taylor, 56, an H.I.V. advocate in Palm Springs, Calif., who enrolled in dozens of trials after his diagnosis in 1982.

Now, 30 years later, “it’s the same group of people, who understand the value of clinical trials.”

Gay men have formed strong support networks that alert potential participants to clinical trials, and they often live in cities where the research is conducted.

By contrast, women with H.I.V. tend to be isolated, and may not advocate for themselves. They may need help with child care or transportation, or be more comfortable with female doctors — accommodations few trials offer.

For women of color, there is an additional hurdle: mistrust resulting from a long history of exploitation by medical researchers. “It’s a lot of stigma still in our community around research,” said Ublanca Adams, 60, who is living with H.I.V. in Concord, Calif.

Scientists do not seem to know how to gain that trust, she said: “How information is given out to our community and our people is just not in a way to be inclusive, nor is it inviting.”

Ms. Adams said she has enrolled in a few observational studies, but does not trust scientists enough to participate in tests of a treatment or cure.

In the rare cases where scientists go the extra mile to enroll women, they face additional scrutiny from the Food and Drug Administration. (The agency has strict rules for including women of childbearing age.)