In a prospective study of morbidity associated with Chlamydia trachomatis infections during pregnancy, we isolated C trachomatis from the endocervix of 18 (6.7%) of 268 women examined before 19 weeks' gestation. Infected women were significantly younger than noninfected women, and significantly more often unmarried, supported by public assistance, and pregnant for the first time. Among women followed up from 19 weeks' gestation until delivery, the mean duration of gestation was significantly shorter for those with antepartum chlamydial infection. Stillbirth or neonatal death occurred in six (33%) of the 18 pregnancies of infected women compared with eight (3.4%) of the 238 pregnancies of noninfected women followed up from the 19th week of gestation through delivery. Stillbirth or neonatal death occurred ten times more often among Chlamydia-infected women than among uninfected controls matched for age, marital status, socioeconomic status, pregnancy order, and race.