OBJECTIVE: Our purpose was determine whether there are ethnic differences in vaginal colonization with organisms reputed to be associated with preterm delivery. STUDY DESIGN: In seven urban medical centers in the United States from 1984 to 1989, 13,747 predominantly low-socioeconomic-status women at 23 to 26 weeks' gestation were grouped according to ethnic origin. Each had a vaginal culture for multiple organisms and Gram's stain and pH test to diagnose bacterial vaginosis. Multivariate analyses with adjustment for maternal age, insurance status, marital status, smoking history, education level, age at first sexual intercourse, and number of male partners in the past year were also performed to determine the odds ratio for a black woman versus a white woman having a positive test result. RESULTS: For nearly every organism studied, Asian-Pacific Islander and white women had the lowest percentages of positive test results whereas black women had the highest. When black and white women are compared, black women were more likely to be colonized with Chlamydia trachomatis, 16% versus 5%; Neisseria gonorrhoeae, 2.5% versus 0.4%; Bacteroides sp., 25% versus 14%; bacterial vaginosis, 23% versus 9%; and the four other organisms evaluated. Adjustment for potential confounders including certain health behaviors did not markedly change the risk of a black woman having a positive test result, with the odds ratios for colonization with each organism ranging from 1.8 to 6.4. CONCLUSIONS: Highly significant differences exist in vaginal colonization patterns among women of different ethnic groups, with the highest rates of potentially pathogenic organisms observed in black women and the lowest in Asian-Pacific Islander women. Differences in health behaviors do not explain the variation in colonization rates.