To determine the usefulness of the vaginal Gram stain as a screen for maternal group B streptococcal carriage, we compared the presence of gram-positive cocci on Gram stain with a cervicovaginal culture in 7755 women at 23-26 weeks' gestation and in 1452 women at delivery. Group B streptococci were isolated from 18.4% of women at 23-26 weeks and 14.9% of women at delivery. The sensitivity, specificity, positive predictive value, and negative predictive value of the Gram stain were 28, 69, 17, and 81%, respectively, in mid-gestation and 34, 72, 18, and 86%, respectively, at delivery. The presence of gram-positive cocci on Gram stain was strongly associated with the isolation of Gardnerella vaginalis and with the presence of bacterial vaginosis. We conclude that most gram-positive cocci seen on Gram stain are probably anaerobes or micrococci and that the vaginal Gram stain is neither sensitive nor specific enough to be of use as a tool in the diagnosis of maternal group B streptococcal carriage.