Objective: To determine the associations between the frequency of unprotected vaginal sex (UVS) and female adolescents' perceptions, particularly their perceptions of relationship dynamics. Design: Cross-sectional study of 522 African American female adolescents enrolled in a sexually transmitted disease (STD) and human immunodeficiency virus prevention intervention trial. Setting and Participants: A volunteer sample of adolescents recruited from neighborhoods characterized by high rates of unemployment, substance abuse, violence, and STDs; 28% tested positive for STDs as assessed by DNA amplification or culture. Main Outcome Measure: Frequency of UVS assessed by interview using a 6-month recall period. Results: Among adolescents having steady relationships, those spending more time with their boyfriends and having longer relationships reported a significantly greater frequency of UVS. Other significant correlates included perception of more girlfriends using condoms, no history of STDs, stronger normative beliefs favoring male decision making in relationships, greater pregnancy worry, and greater perceived invulnerability to STDs. For adolescents reporting casual relationships, personal barriers to condom use, no history of STDs, and reporting that their boyfriends typically decide when to have sex were associated with more frequent UVS. Conclusions: Adolescents' perceptions, particularly their perceptions of relationship dynamics, played an integral role in explaining female adolescents' frequency of UVS with both steady and casual partners. Female adolescents in steady relationships differ from those in casual relationships relative to their prevention needs. These findings have implications for clinic- or community-based STD and human immunodeficiency virus prevention programs.