The authors sought to quantify the overall and race/ethnic-specific relations between prepregnancy body mass index and both preterm birth and vaginal inflammation. Data from a cohort of 11,392 women who enrolled in the multicenter Vaginal Infections and Prematurity Study (1984-1989) at 23-26 weeks' gestation were used. Compared with a prepregnancy body mass index of 22, a body mass index of 16 increased the risk of preterm birth by 90% (odds ratio = 1.9, 95% confidence interval (CI): 1.5, 2.6), and a body mass index of 18 increased the risk by 40% (odds ratio = 1.4, 95% CI: 1.2, 1.7). Ethnicity substantially modified the magnitude of the body mass index effect and the shape of the preterm birth risk curve, with underweight having a greater impact on preterm birth among Blacks and Hispanics than among Whites. Low body mass index increased the risk of a high level of neutrophils (> 5 per oil immersion field) and a high vaginal pH measurement (> or = 5.0) among Black women; for a body mass index of 16 versus 22, the odds ratio = 1.7 (95% CI: 1.1, 2.6). Compared with Black women with a body mass index of 22, Blacks with a body mass index of 16 had a 1.7-fold increased risk for a high level of neutrophils and a high vaginal pH measurement, while those with a body mass index of 18 had a 1.3-fold increased risk.