With young people accounting for a disproportionate amount of new HIV infections, San Francisco is preparing to open its first health clinic targeted at gay adolescents and young adults.

The clinic, called Dimensions, is a collaboration of the city's Department of Public Health and several nonprofit agencies. It will open July 2, offering medical and mental health services to a population overlooked by mainstream health agencies.Similar clinics operate in Boston, Los Angeles and New York.

The San Francisco center will offer prevention and testing for the human immunodeficiency virus, which causes AIDS. Half of all new HIV infections today occur in people under 25. But they account for only 17 percent of those getting tested, said Steven Tierney, executive director of Health Initiatives for Youth, a nonprofit organization that helped develop the clinic.

"They don't get tested because they're not hooked up to health care or have never had good experiences" with health care, Tierney said.

Kevin Gogin, head of gay and lesbian youth support services for the San Francisco schools, said the clinic would be a vital for gay youths with nowhere else to turn.

"A lot of family physicians are not going to be comfortable or knowledgeable about some of the issues these young people will be bringing in," he said.

But the health concerns of gay youths go beyond AIDS prevention. Mainstream agencies tend to focus on AIDS and overlook other health needs of gay adolescents and young adults, health professionals at the Dimensions clinic said.

Tierney said estimates of the number of gay, lesbian, bisexual and transgender youths in San Francisco range from 6,000 to 10,000. The city has a population of more than 720,000.

The idea for the clinic resulted from a survey last summer of 200 gay, lesbian, bisexual and transgender people ages 12 to 25.

Conducted by the Health Initiatives for Youth, the survey found that access to health care, therapy and substance-abuse treatment were leading concerns for gay adolescents and young adults.

Many of them are not in the habit of seeking health care or counseling. Those who do have often had negative experiences.