In this retrospective study, the efficacy of screening for and treating cervical Chlamydia trachomatis infection was evaluated in a pregnant population at increased risk for chlamydial infection. Over a 2 1/2-year period, 5.75% (338) of the 5,875 women tested were found to be infected with this organism. Of the 323 women patients available for follow-up, 76% (244) were successfully treated and 24% (79) remained infected throughout their pregnancies. Forty (12%) patients became infected during pregnancy, while 26 (8%) were reinfected during pregnancy, despite treatment with erythromycin. Twenty-seven (8%) patients had their first antenatal visit and cervical swab less than a week before delivery. The gestational age at which the first cervical chlamydial swab was obtained was significantly more advanced in patients who remained infected (30.23 +/- 6.2 weeks) than those who were successfully treated (22.15 +/- 7.66 weeks; P = 0.00001). The data suggest that in a pregnant population considered to be at increased risk for C trachomatis infection: (1) there is a subgroup of patients with a high risk of remaining infected or becoming reinfected with C trachomatis during pregnancy despite treatment with erythromycin and (2) repeated prenatal testing and treatment of those infected is necessary to detect and eradicate maternal chlamydial infection.