In a prospective study of chlamydial and mycoplasmal infections in pregnancy, Chlamydia trachomatis occurred in 8.0%, Mycoplasma hominis in 23.5%, and Ureaplasma urealyticum in 72.3% of 1,365 enrollees. By multivariate analysis, C trachomatis was correlated with lower socioeconomic status, age 23 years or younger, and 12 years or less of schooling. Ureaplasma urealyticum was correlated with age 23 years or younger and lower socioeconomic status. Mycoplasma hominis was correlated with more than one recent sexual partner, first intercourse at age 17 years or younger, and higher socioeconomic status. These cervical infections did not predict low birth weight, abortion, stillbirth, prematurity, or premature rupture of membranes. Only M hominis predicted endometritis/fever after vaginal delivery (relative risk, 7.3). IgM-seropositive C trachomatis-infected women had more low-birth-weight infants and more premature rupture of membranes than either IgM-negative C trachomatis-infected women or C trachomatis culture-negative women. Thus, only certain subgroups of infected women may experience adverse pregnancy outcomes.