We evaluated the presence of polymorphonuclear cells on endocervical Gram stains and clinical indices of cervicitis as predictors of Chlamydia trachomatis infection in 193 sexually active female adolescents; 148 (77%) subjects had no complaints. C. trachomatis was isolated from 34 (18%) subjects. The incidence of C. trachomatis differed by race: it was isolated from 30% of black subjects and 7% of nonblack subjects. Polymorphonuclear cells on Gram stains were found to be associated with the presence of C. trachomatis (chi 2 = 33.2, p less than 0.0001). The sensitivity, specificity, and positive predictive value for polymorphonuclear cells to predict the presence of C. trachomatis were 91% (31/34), 65% (103/159), and 36% (31/87), respectively. When examined by ethnicity, polymorphonuclear cells remained significantly associated with C. trachomatis for black subjects (chi 2 = 24.3, p less than 0.001) but not for nonblack subjects (chi 2 = 2.49, p less than 0.1). Mucopurulent discharge, friability, or erythema-edema of ectopy were not reliable indicators for C. trachomatis. Endocervical Gram stains alone are useful to identify populations at risk for C. trachomatis infection. Caution must be advised when interpreting data from different population groups because of the variability in C. trachomatis prevalence.