Objective: To determine the time between first intercourse and first sexually transmitted infection (STI) with. Chlamydia trachomatis, Neisseria gonorrhoeae, or Trichomonas vaginalis and time between repeated infections. Design: Observational study. Setting: Three adolescent medicine clinics. Participants: A cohort of 386 urban young women aged 14 to 17 years at enrollment. Main Outcome Measures: Age at first intercourse; organism-specific interval between first intercourse and first STI diagnosis; interval between repeated infections; and age at first STI test prior to study participation. Results: Participants had first intercourse at a young age (first, second, and third quartiles were 13, 14, and 15 years of age, respectively). By age 15 years, 25% of the women acquired their first STI, most often C trachomatis. Median interval between first intercourse and first STI diagnosis was 2 years. Within 1 year of first intercourse, 25% had their first C trachomatis infection. Repeated infections were common; within 3.6, 6, and 4.8 months, 25% of the women with prior C trachomatis, N gonorrhoeae, and T vaginalis infection were reinfected with the respective organisms. Considerable delay in STI testing was found for those who began sex at a younger age. The median interval between first sex and first test were 4.9, 3.5, 2.1, 1.8, and 1.2 years for those who had first sex at ages 10, 11, 12, 13, and 14 years, respectively. Conclusions: Timely screening and treatment are important for prevention of STI sequelae. For urban adolescent women, STI screening (especially for C trachomatis) should begin within a year after first intercourse and infected individuals should be retested every 3 to 4 months. ©2009 American Medical Association. All rights reserved.