To determine the prevalence of Chlamydia trachomatis and Neisseria gonorrhoeae endocervical infections in a group of adolescent women, gynecologic histories and evaluations were done on symptomatic and asymptomatic adolescent women presenting for pelvic examinations in an urban, hospital-based, adolescent clinic. C. trachomatis and N. gonorrhoeae cultures and three nonculture diagnostic tests for chlamydia (two ELISA assays and one DNA-probe) were performed on each patient. Patients were 12 to 21 years of age (mean 17.0); 53% were African-American, and 47% were Caucasian. Two hundred twenty-eight women were asymptomatic and 249 women had symptoms. There were 64 cases of chlamydia and 19 cases of gonorrhea; an additional 11 patients had both chlamydia and gonorrhea. Approximately one third of patients with chlamydia, gonorrhea, or both had asymptomatic disease an important reservoir of infection.