Study subjects were limited to Covenant House clients who underwent an initial medical evaluation between October 1, 1987, and December 31, 1989. Clients seen for annual or followup visits were excluded to avoid repeat testing of the same persons over time. A risk-assessment component was added in May 1988. As of December 1989 a total of 2,667 adolescents had been tested, and 142 (5.3 percent) were found to be HIV-seropositive (males, 6.0 percent; females, 4.2 percent). The seroprevalence rate increased from 1.3 percent for 15-year-olds to 8.6 percent for 20-year-olds. Hispanics had the highest seroprevalence rate (6.8 percent), followed by non-Hispanic whites (6.0 percent) and non-Hispanic blacks (4.6 percent). HIV seropositivity was associated with intravenous drug use, male homosexual/bisexual activity, prostitution, and history of another sexually transmitted disease. The high prevalence of HIV infection in this selected population indicates the immediate need for AIDS prevention programs for adolescents. 3 tables and 5 references