The objective of this study was to examine the associations between lower genital tract infection, racial group and preterm birth in the Vaginal Infections and Prematurity Study, a large prospective cohort study conducted between 1984 and 1989. This analysis included 11 910 women enrolled at 23-26 weeks' gestation with equal representation from self-identified African American, Hispanic and white women. Subjects were screened for Chlamydia trachomatis, Trichomonas vaginalis and bacterial vaginosis at study entry, and their pregnancy outcomes were ascertained after delivery. The primary outcome of interest was preterm delivery of a low-birthweight infant (<37 weeks and <2500 g). The associations between lower genital tract infection and preterm delivery of a low-birthweight infant were examined within each racial group, with adjustment for potential confounders using multivariable logistic regression. In this cohort, 6.4% of African Americans, 3.8% of Hispanics, and 4.4% of whites had a preterm delivery of a low-birthweight infant (P < 0.001). Lower genital tract infection was significantly associated with preterm delivery of a low-birthweight infant among African Americans, but not among other racial groups. The proportion of preterm birth associated with lower genital tract infection was 21% among African Americans and 5% among whites. The increase in infection-associated preterm birth among African Americans appears to be related both to an increased prevalence of lower genital tract infection, and also to an increased risk of preterm delivery of a low-birthweight infant in the context of lower genital tract infection.