Sixty-eight sexually active and 52 virginal adolescent girls were evaluated for six sexually transmissible infectious agents: Gardnerella vaginalis, Ureaplasma urealyticum, Mycoplasma hominis, Chlamydia trachomatis, Trichomonas vaginalis, and Neisseria gonorrhoeae. There were significant differences between sexually active and virginal girls with respect to the prevalence of isolation of U urealyticum (75% v 33%, P less than .005), M hominis (27% v 10%, P less than .05), and C trachomatis (19% v 2%, P less than .025) but not for G vaginalis (34% v 17%, P = .09). N gonorrhoeae and T vaginalis were isolated exclusively from sexually active girls, but their low prevalence (6% and 9%, respectively) made the difference statistically insignificant (P = .2 and .06, respectively). Race, current v previous sexual activity, multiple sexual partners, oral contraceptive use, and concurrent isolation of another organism did not identify those at increased risk for chlamydial isolation. Such girls were significantly more likely to have inflammatory Papanicolaou smears (36% v 10%, P less than .05) and excessive WBC in their vaginal secretions (50% v 19%, P = .05). The data support the contention that C trachomatis, N gonorrhoeae, and T vaginalis are organisms that are predominantly acquired via sexual routes. Significant nonsexual modes of transmission are supported by the data for the genital mycoplasmas and G vaginalis. Finally, a history of sexual activity in an adolescent female warrants specific diagnostic testing for Chlamydia.