Pregnancy outcomes of 244 women, successfully treated with erythromycin ethylsuccinate for cervical chlamydial infection, were compared with those of 79 chlamydia-positive pregnant women, who failed to respond to treatment, and 244 chlamydia-free control women, who were not treated. The three groups were at high risk for infection with Chlamydia trachomatis. The frequencies of premature rupture of the membranes, premature contractions, and small-for-gestational-age infants were significantly lower in the successfully treated patients when compared with those of the chlamydia-positive patients, but were not significantly different when compared with those of the control patients. These data indicate that in a pregnant population at high risk for infection with C trachomatis, repeated prenatal chlamydial testing, plus successful erythromycin treatment, can significantly reduce certain adverse effects on pregnancy outcome.