Objective: To prospectively determine (using an 18-month follow-up period) the association between African American female adolescents' perceptions of parental monitoring and their acquisition of biologically confirmed infection with Chlamydia trachomatis, Neisseria gonorrhoeae, and Trichomonas vaginalis. Design: A prospective cohort study of 217 African American female adolescents enrolled in the control arm of a randomized trial of a human immunodeficiency virus prevention intervention program. Setting and Participants: A volunteer sample of adolescents (aged 14-18 years) recruited from low-income neighborhoods characterized by high rates of unemployment, substance abuse, violence, and sexually transmitted diseases. Main Outcome Measures: Adolescents provided 2 self-collected vaginal swab specimens. One was tested for C trachomatis and N gonorrhoeae DNA with ligase chain reaction. The other was used to inoculate culture medium for T vaginalis. Identical assay procedures were repeated at the 6-month, 12-month, and 18-month follow-up intervals. Results: Adjusted odds ratios indicated that adolescents who perceived infrequent parental monitoring at baseline were 1.8 (95% confidence interval, 1.01-3.21) and 2.4 (95% confidence interval, 1.22-4.87) times more likely to acquire chlamydia or trichomoniasis, respectively, compared with their counterparts who perceived greater levels of monitoring. Similarly, adolescents who perceived infrequent parental monitoring were 2.1 (95% confidence interval, 1.16-3.74) times more likely to test positive for a sexually transmitted infection during the course of the 18-month follow-up period. Conclusions: Adolescents' perceptions of their parental-monitoring levels predicted subsequent acquisition of biologically confirmed chlamydia and trichomoniasis infections. These findings suggest that expanded efforts leading toward effective clinic- and community-based sexually transmitted infection intervention programs involving parents may be warranted.