Ms. Tobin said her organization heard frequently from transgender people who had faced discrimination in health care — or were simply afraid to seek medical care.

Mara Youdelman, a lawyer with the National Health Law Program, added that, for example, someone who was born female, identifies as a transgender male but has not had gender reassignment surgery, might be denied insurance coverage for a Pap smear without the proposed rule.

“There was nothing to prohibit the exclusion of those services by an insurer,” Ms. Youdelman said.

Ms. Youdelman said the proposed rule was significant because it “puts the meat on the bone” of a provision that for the first time prohibits sex discrimination in health care. For the other groups protected under the provision, Section 1557 of the Affordable Care Act, it merely “reiterates longstanding prohibitions” on discrimination that other laws, including Title VI of the Civil Rights Act, already provide.

“For the first time, you can’t discriminate in health care because of someone’s sex,” she said. “And the regulation that we got today greatly elucidates what’s expected.”

Though the proposed regulation spells out more specifics than the two-paragraph provision in the health law, Jocelyn Samuels, the director of the Office for Civil Rights at the Department of Health and Human Services, said that her office had already been responding to complaints and negotiating settlements when they determined discrimination had taken place.

For example, Ms. Samuels said her office had recently resolved a complaint that a transgender patient filed against Brooklyn Hospital Center, alleging she had been denied a room assignment consistent with her gender identity. Ms. Samuels also pointed to a case in which a male victim of domestic violence was denied services at a hospital because, she said, he did not fit “the traditional profile” of a domestic violence victim.

“Sadly, we have ample evidence that there continues to be a persistent problem with discrimination in the health care industry,” Ms. Samuels said during a telephone briefing with reporters. But while her office receives thousands of complaints each year about discrimination in health care, she added, the proposed rule “provides a very valuable tool for us to be able to appropriately address them.”