We investigated the relationship of route of delivery and of antepartum Chlamydia trachomatis cervical infection to selected forms of puerperal infectious morbidity, including intrapartum fever, or early (less than 48 hours) postpartum fever not attributable to infections of sites outside the uterus and late (48 hours to 6 weeks) postpartum endometritis. Infectious morbidity occurred in 27 (44%) of 62 women who underwent cesarean section and in 33 (10%) of 329 who underwent vaginal delivery (p less than 0.001). Postsection infectious morbidity was not correlated with C. trachomatis infection and was largely limited to early postpartum fever. Among women who underwent vaginal delivery, infectious morbidity occurred in 10 (34%) of 29 women with and in 23 (8%) of 300 without C. trachomatis infection (p less than 0.001), and chlamydial infection was associated only with intrapartum fever and late postpartum endometritis. Separate matched case-control analyses confirmed that cesarean section was associated with an increased risk of early postpartum fever (p = 4 x 10(-8)); whereas among women who underwent vaginal delivery, antepartum C trachomatis infection was associated with an increased risk of development of intrapartum fever or late postpartum endometritis (p = 0.002).