We studied associations of Mycoplasma hominis, Ureaplasma urealyticum, and Chlamydia trachomatis genital infections with pregnancy outcomes, controlling by logistic and multiple linear regression for known risk factors and for the presence of the other two infections. A sample of 1204 Navajo women enrolling for prenatal care had endocervical C trachomatis, M hominis, and U urealyticum cultures and serum samples taken at enrollment and when possible after 30 weeks. Low birth weight (less than 2500 g) was associated with M hominis infection among women with a history of spontaneous abortion. Mycoplasma hominis infection was also associated with postpartum endometritis, but only among women undergoing a cesarean section (odds ratio, 4.7; 95% confidence intervals, 1.22 to 18.3). Although women with recent C trachomatis infection (IgM titer greater than 1:32 on either sample or IgG seroconversion) were at greater risk of low birth weight (19% [3/16]) than women with chronic infection (4.5% [6/133]; relative risk, 4.2), this subgroup at risk was small (11% of women with classifiable C trachomatis infection). Mycoplasma hominis and C trachomatis infections may be important preventable causes of adverse pregnancy outcomes in identifiable subgroups of women.