The study was part of a national HIV surveillance project coordinated by the Centers for Disease Control and Prevention. The target population for the San Francisco portion of the study included homeless and runaway adolescents and young adults who sought health care at the Cole Street Clinic and the Larkin Street Youth Center. During the study period, 1,033 clients were approached to participate and were screened for eligibility; 847 young people met eligibility criteria, and 334 of eligible clients consented to participate, provided blood specimens, and completed interviews. Findings revealed gay, lesbian, and bisexual young people reported higher levels and earlier onset of sexual and drug-using behavior compared to their heterosexual counterparts. Further, gay, lesbian, and bisexual young people were at exceptionally high risk for HIV infection. Although gay, lesbian, and bisexual participants were older than heterosexual participants, an examination of the age at onset indicated risk behaviors began earlier and achieved higher levels for all ages. Of the participants, 27 percent of males and 17 percent of females described themselves as gay, lesbian, or bisexual, a higher proportion than expected in the general population. The authors conclude HIV and drug use health education messages should explicitly address sexual orientation by or before age 10 to achieve the greatest chance of influencing risk behaviors before they become well-established. 34 references, 1 note, 3 tables, and 2 figures