Cultures for Chlamydia trachomatis were obtained from 1012 teenage girls attending an adolescence clinic mainly for family planning; C. trachomatis was isolated from 174 (17.2%). The proportion of chlamydia-positive girls varied between 15.7% and 28.5% depending whether or not there were symptoms and signs of infection. Neither the history nor the finding at pelvic examination offered conclusive evidence for or against the presence of an infection with C. trachomatis. Of the chlamydia-positive, untreated, asymptomatic girls, 17.5% developed symptoms of a genital infection within 3 months. There was a statistically significant difference in the proportion of positive cultures by the two investigators, suggesting that the procedure for specimen collection is of great importance.