In an effort to prevent perinatal acquisition of Chlamydia trachomatis, we offered treatment with erythromycin ethylsuccinate (400 mg four times a day for seven days, given at 36 weeks' gestation) to 184 pregnant women with cervical chlamydial infections. Thirty-two women refused treatment; 24 of their infants were followed and served as the controls. Therapy was discontinued by 5 of 10 women who had gastrointestinal disturbances. Forty-seven women who completed therapy refused infant follow-up; in four (9 percent) of these women, therapy had failed to eradicate the infection. Sixty women and 59 infants completed the entire protocol; 55 (92 percent) of the women had negative cultures for chlamydia at follow-up. Chlamydial infection developed in 4 (7 percent) of the 59 infants of treated mothers, as compared with 12 (50 percent) of the 24 infants of untreated mothers; this difference was significant (P less than 0.001). With a success rate of 92 percent (98 of 107 patients) in treating maternal infection and with a relatively low intolerance rate (3 percent; 5 of 152), this regimen appears to be an effective, although not ideal, therapy for chlamydial infection in pregnant women. We conclude that in settings in which the prevalence of chlamydia infection is high, a routine program of screening pregnant women for cervical C. trachomatis, followed by treatment of those infected, would be cost effective and would reduce infant morbidity.