Objectives: To screen and treat chlamydial infection in pregnant women in order to assess the effects of therapeutic intervention on the outcome of Chlamydia trachomatis-infected pregnancy. Methods: Enrolled in the study were 350 women in the first to third trimesters of pregnancy. Endocervical swabs were collected for C. trachomatis diagnosis by DFA and PCR. A few STD infections, viz.: Neisseria gonorrhoeae, Candida spp., bacterial vaginosis, Trichomonas vaginalis and Treponema pallidum were ruled out. After excluding 22 patients infected with other sexually transmitted infections, a cohort of 328 pregnant women comprised the study population. Anti-chlamydial treatment (viz.: oral therapy with erythromycin stearate, 500 mg 4 times daily for 7 days) was given to 17 women (group I) and their partners. Fifteen patients of group I were retested by DFA and PCR assay for C. trachomatis infection 2 weeks following therapy. Other Chlamydia-positive patients (n=42) who were lost to follow-up were classified as untreated positive cases (group II) while group III included C. trachomatis negative cases (n=269). Data on obstetric outcome were recorded in a total of 164 cases. Statistical comparison of the data were done using the χ2-test and means were compared using Student''s t-test. Results: Among the 350 pregnant women enrolled initially for the study, C. trachomatis positivity was found to be 18.8% (n=66) in the endocervix by DFA and PCR assay. Co-infection with Candida spp., bacterial vaginosis, T. vaginalis and T. pallidum was 2.0%, 1.7%, 1.7% and 0.5%, respectively. None of the pregnant women was infected with N. gonorrhoeae. Pregnant women at an increased risk of chlamydial infection included those who had multiple births and were in second trimester of pregnancy. Fifteen patients of group I became Chlamydia-negative following treatment. Data on obstetric outcome were recorded in 11, 26 and 127 patients of groups I, II and III, respectively. The mean duration of gestation for premature deliveries was found to be significantly higher in group I in comparison with group II [35.5 vs. 33.1 weeks (P<0.05)], thereby showing an improved effect of treatment on pregnancy outcome. The mean of low birth weight births was higher in group I compared with group II (2200.0 vs. 2113.3 g), however, this was statistically non-significant. Stillbirths were significantly higher among group II in comparison with group III [11.5% vs. 4.7% (χ2=1.79; P<0.5)]. No stillbirths were recorded in patients who had taken anti-chlamydial treatment. Conclusions: Our findings suggest that routine screening and treatment of C. trachomatis infection in pregnant women, especially those in high risk groups, should be mandatory to reduce the adverse effects on obstetric outcome. [Copyright &y& Elsevier]