Twenty-one percent of a sample of inner-city junior high school students were found to be sexually active (n = 403). Only 31% of them reported a single lifetime sexual partner, 25% reported two partners, and 43% reported three or more partners. Logistic regression analysis evaluated the influence of demographic, psychosocial, and behavioral factors on the adolescents' lifetime number of sex partners. Respondents whose sexual debut occurred before age 13 years were nine times more likely to report three or more sex partners compared with those whose first sexual intercourse was at age 15 or 16 years, blacks were four times more likely than non-Hispanic whites to report three or more sex partners; and males were four times as likely as females to report this number of sexual partners. Factors not independently associated with the number of sex partners included: age, Asian or Hispanic ethnicity, human immunodeficiency virus (HIV) knowledge, self-efficacy (belief that one can protect oneself from the virus), condom use, and alcohol and drug use. We conclude that a significant proportion of school-based middle adolescents are sexually active and that most of these are at risk for contracting HIV because of behaviors such as having multiple sexual partners. Topics often stressed in school-based HIV education, such as factual knowledge about HIV, avoiding drugs and alcohol, and condom use are not associated with adolescents' choice about their number of sex partners. Intervention programs will have to identify and then target each specific HIV risk behavior and its motivations in order to reduce adolescents' risks of contracting and transmitting the disease.