Among 100 randomly selected nonmenstruating women attending a clinic for sexually transmitted diseases, we assessed objective criteria for the clinical diagnosis of mucopurulent cervicitis. Visualization of yellow mucopurulent endocervical secretions on a white swab and the presence of 10 or more polymorphonuclear leukocytes per microscopical field (at a magnification of 1000) in satisfactory gram-stained endocervical smears were independently correlated with cervical Chlamydia trachomatis infection. Neither finding correlated with gonorrhea or genital herpes, although herpes caused characteristic cervical ulcerations. C. trachomatis was isolated from the cervix of 20 of 40 women with mucopurulent cervicitis but of only 2 of 60 without it. The overall prevalence of mucopurulent cervicitis among women attending the clinic (40 per cent) exceeded that of nongonococcal urethritis among men in the same clinic, and the prevalence of C. trachomatis infection was higher in mucopurulent cervicitis than in nongonococcal urethritis, a condition that is conventionally treated with tetracyclines. These findings support recommendations for the treatment of mucopurulent cervicitis and should guide the selective use of confirmatory diagnostic tests for C. trachomatis infection.