Chlamydia trachomatis infection is one of the most common sexually transmitted diseases (STD) among adolescents and is one of the most common causes of perinatal infection. We have screened 267 young nulliparous adolescent girls (12-17 years old) for cervical C. trachomatis infection during their pregnancy using tissue culture methods. The population was a cohort of pregnant teenagers who had limited sexual exposure (mean duration of sexual activity, 14.3 months, mean number of lifetime partners, 2.0 +/- 1.5). The initial screening was done at enrollment to a prenatal care clinic, and the third trimester examination at 32 to 36 weeks of gestation. According to clinical indications, additional interim STD examinations were carried out. The cervical C. trachomatis infection rate was 18.7% (50/267) at intake and 7.9% (14/178) in the third trimester. Three of the 40 interim STD examinations also were positive for cervical C. trachomatis infection. Prenatally, 64 of 267 (24.0%) girls were positive for C. trachomatis at one or more times. The repeat culture positive rate for C. trachomatis was 7.8% (17/218), and 14 of those that were negative on the initial screening were positive on the subsequent screening. One or more types of treatable STDs (chlamydial, gonorrhea, syphilis, or trichomonal) were identified in 38.6% (103/267) of the study population during pregnancy. These findings suggest that high-risk sexual behavior may continue in teen pregnancy, and repeated prenatal chlamydial and other STD screening and counseling are indicated in this population.