• To determine the prevalence of infection with Chlamydia trachomatis in young girls with the same socioeconomic background from New York City, 186 sexually active female adolescents (age range, 12 to 17 years; mean age, 15.5 years) were screened. One third of the patients were pregnant. Papanicolaou smears, endocervical cultures for C trachomatis and Neisseria gonorrhoeae, and syphilis serologic tests were obtained prospectively. Chlamydia trachomatis was isolated from 10.2% of the subjects, and N gonorrhoeae was isolated from 9.7% of the subjects; 3.2% of the subjects had syphilis. At least one sexually transmitted disease was found in 17.2% of the subjects. Eight (44%) of 18 patients with N gonorrhoeae also harbored Chlamydia. The high rate of multiple infection should have significant implications with regard to treatment. High isolation rates of C trachomatis and other sexually transmitted pathogens suggested that routine screening may be warranted in even the very young, sexually active female adolescent, and especially in pregnant girls.