Risk factors for cervicovaginal group B streptococcal colonization at 23-26 weeks' gestation were studied in 7742 women participating in the Vaginal Infections and Prematurity study. The prevalence of group B Streptococcus was 18.6%, and was greatest in (predominantly Caribbean) Hispanics from New York City, followed by blacks, whites, and other (predominantly Mexican) Hispanics. Group B Streptococcus was more common among older women and women of lower parity, and less common among women living with their partner compared with those living alone. Current smoking was associated with a decreased risk of colonization, and group B Streptococcus was less common among women with more education. Increased risk was seen only with extreme increases in sexual activity including both frequent intercourse and multiple partners during the previous year. The risk of colonization was greater when there was concurrent colonization with Candida sp, but group B Streptococcus was not associated with carriage of Chlamydia trachomatis, Ureaplasma urealyticum, Trichomonas vaginalis, and Mycoplasma hominis. External genital erythema and scaling, purulent vaginal discharge, and pH greater than 5 were associated with increased colonization. Although these associations can raise the clinical index of suspicion for group B streptococcal colonization in a given patient, the study data did not enable us to select a small group of women with a very high probability of colonization. We conclude that selective screening is not useful in detecting group B streptococcal colonization in pregnancy.