Several studies have suggested that pregnant women infected with Trichomonas vaginalis may be at increased risk of an adverse outcome. To evaluate prospectively the association between T. vaginalis and risk of adverse pregnancy outcome in a large cohort of ethnically diverse women. At University-affiliated hospitals and antepartum clinics in five United States cities, 13,816 women (5,241 black, 4,226 Hispanic, and 4,349 white women) were enrolled at mid-gestation, tested for T. vaginalis by culture, and followed up until delivery. The prevalence of T. vaginalis infection at enrollment was 12.6%. Race-specific prevalence rates were 22.8% for black, 6.6% for Hispanic, and 6.1% for white women. After multivariate analysis, vaginal infection with T. vaginalis at mid-gestation was significantly associated with low birth weight (odds ratio 1.3; 95% confidence interval 1.1 to 1.5), preterm delivery (odds ratio 1.3; 95% confidence interval 1.1 to 1.4), and preterm delivery of a low birth weight infant (odds ratio 1.4; 95% confidence interval 1.1 to 1.6). The attributable risk of T. vaginalis infection associated with low birth weight weight in blacks was 11% compared with 1.6% in Hispanics and 1.5% in whites. After considering other recognized risk factors including co-infections, pregnant women infected with T. vaginalis at mid-gestation were statistically significantly more likely to have a low birth weight infant, to deliver preterm, and to have a preterm low birth weight infant. Compared with whites and Hispanics, T. vaginalis infection accounts for a disproportionately larger share of the low birth weight rate in blacks.