We used multiple logistic regression to analyze microbiological and clinical correlates of inflammatory and epithelial cell changes on Papanicolaou-stained cervical smears in patients from a sexually transmitted disease clinic. Among randomly selected patients, increased numbers of histiocytes and polymorphonuclear leukocytes and the presence of transformed lymphocytes were associated with Chlamydia trachomatis infection, while increased lymphocytes were associated with Trichomonas vaginalis infection; minimal squamous atypia was associated with yeast infection; and moderate squamous atypia and koilocytosis were associated with cervical condylomata visualized by colposcopy. Among patients referred for cervicitis, C trachomatis infection was also associated with reactive or atypical metaplastic cells. Distinct inclusions were seen by Papanicolaou smear in only 22% of C trachomatis infections. In randomly selected patients, however, the presence of transformed lymphocytes or increased histiocytes had a sensitivity of 95%, a specificity of 75%, and a positive predictive value of 50% in relation to isolation of C trachomatis, and could therefore be used for selection of patients for confirmatory testing for C trachomatis infection.