Interventions with Puerto Rican adolescents should focus on the use of contraception, and interventions with African-American adolescents should address reducing the number of sexual partners and lengthening the time before becoming sexually active with a new partner. The study shows that many reproductive-health behaviors, awareness, and social context differed between African-American and Puerto Rican adolescent females living in the same geographical area. These differences remained after controlling for indicators of family and community socioeconomic status. Puerto Rican adolescents reported more sexual activity and less use of protection than African-American adolescents; however, African-American adolescents had more sexual partners and were in relationships of shorter duration. Over time, the higher number of sexual partners may increase African-American adolescents' risk for sexually transmitted diseases (STDs). Puerto Rican female adolescents, on the other hand, may be at greater risk of pregnancy because of unprotected sex. Although Puerto Rican adolescents reported greater awareness of STDs, they were still less likely to use protection, and they had a more negative attitude toward condoms. Puerto Rican adolescents reported having more friends who had become pregnant, but fewer friends who were sexually active. Participants were adolescent females involved in a prospective study of HIV/STD risk behavior between June 1998 and March 2000. Eligibility requirements included being between 14 and 19 years old, having had sexual intercourse at least once, and not yet having any children. Participants completed four face-to-face interviews during 18 months. The variables measured pertained to demographics, reproductive-health behaviors, reproductive-health knowledge and attitudes, reproductive-health peer/partner social context, and reproductive-health family social context. 3 tables and 24 references