Chlamydia trachomatis is now recognized as the most common sexually transmitted disease organism in the United States. Although the potential for vertical transmission of C. trachomatis from pregnant women to their infants is well established, the extent to which infection adversely affects pregnancy and causes perinatal complications remains controversial. We report herein the results of a prospective study of 270 pregnant women with endocervical C. trachomatis compared with 270 matched control subjects (age +/- 1 year, race, and socioeconomic status). Among the entire group (n = 540), the rates of pregnancy complications were: premature rupture of the membranes, 54/270 (10%); preterm delivery, 55 (11%); amnionitis, 20 (4%); intrapartum fever, 23 (4.3%); small for gestational age, 76 (14.5%); postpartum endometritis, 31 (6%); and neonatal sepsis, 10 (1.8%). No statistically significant differences were noted between cases and controls for any of these variables. In the subset of women with recent or invasive chlamydial infection, indicated by the presence of IgM antibody against C. trachomatis, preterm delivery occurred in 13/67 IgM-positive versus 8/99 IgM-negative (p = 0.03) cases. Premature rupture of the membranes was present in 13/67 IgM-positive versus 8/99 IgM-negative (p = 0.03).