African American adolescent girls are at high risk for human immunodeficiency virus (HIV) infection, but interventions specifically designed for this population have not reduced HIV risk behaviors. To evaluate the efficacy of an intervention to reduce sexual risk behaviors, sexually transmitted diseases (STDs), and pregnancy and enhance mediators of HIV-preventive behaviors. Randomized controlled trial of 522 sexually experienced African American girls aged 14 to 18 years screened from December 1996 through April 1999 at 4 community health agencies. Participants completed a self-administered questionnaire and an interview, demonstrated condom application skills, and provided specimens for STD testing. Outcome assessments were made at 6- and 12-month follow-up. All participants received four 4-hour group sessions. The intervention emphasized ethnic and gender pride, HIV knowledge, communication, condom use skills, and healthy relationships. The comparison condition emphasized exercise and nutrition. The primary outcome measure was consistent condom use, defined as condom use during every episode of vaginal intercourse; other outcome measures were sexual behaviors, observed condom application skills, incident STD infection, self-reported pregnancy, and mediators of HIV-preventive behaviors. Relative to the comparison condition, participants in the intervention reported using condoms more consistently in the 30 days preceding the 6-month assessment (unadjusted analysis, intervention, 75.3% vs comparison, 58.2%) and the 12-month assessment (unadjusted analysis, intervention, 73.3% vs comparison, 56.5%) and over the entire 12-month period (adjusted odds ratio, 2.01; 95% confidence interval CI, 1.28-3.17; P =.003). Participants in the intervention reported using condoms more consistently in the 6 months preceding the 6-month assessment (unadjusted analysis, intervention, 61.3% vs comparison, 42.6%), at the 12-month assessment (unadjusted analysis, intervention, 58.1% vs comparison, 45.3%), and over the entire 12-month period (adjusted odds ratio, 2.30; 95% CI, 1.51-3.50; P<.001). Using generalized estimating equation analyses over the 12-month follow-up, adolescents in the intervention were more likely to use a condom at last intercourse, less likely to have a new vaginal sex partner in the past 30 days, and more likely to apply condoms to sex partners and had better condom application skills, a higher percentage of condom-protected sex acts, fewer unprotected vaginal sex acts, and higher scores on measures of mediators. Promising effects were also observed for chlamydia infections and self-reported pregnancy. Interventions for African American adolescent girls that are gender-tailored and culturally congruent can enhance HIV-preventive behaviors, skills, and mediators and may reduce pregnancy and chlamydia infection.