The effect of Chlamydia trachomatis on pregnancy outcome and the effect of treatment of positive cervical cultures was studied by culturing 11,544 women for chlamydia at their first prenatal visit. Chlamydia culture was positive in 2433 (21.08%) and prevalence was related to age and race. Of the positive cultures, 1110 were classified as untreated. The untreated group demonstrated a significant increase in the incidence of premature rupture of the membranes and low birth weight and a decrease in survival when compared with either those with positive cultures who received treatment (N = 1323) or those with negative cultures (N = 9111). Screening of populations at high risk of chlamydia is recommended and treatment of chlamydia-positive patients may improve pregnancy outcome.