Abstract 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<0.001). With a success rate of 92 percent (98 of 107 patients) in treating maternal infection an...