Bacteriuria in pregnancy was prospectively studied in 569 women, with specific reference to group B streptococcal infection. Forty-six patients (8%) had bacteriuria, including 14 with group B streptococcal infection; group B streptococci (GBS) were exceeded in frequency only by Escherichia coli. Two thirds of the bacteriuric patients remained asymptomatic. The outcome of pregnancy was studied in 41/46 bacteriuric patients, including all those with group B streptococcal infection. Two pregnancies ended in intrauterine fetal death, and one neonate developed group B streptococcal sepsis; all three complications occurred in the 14 women with group B streptococcal bacteriuria. Diabetes mellitus appeared to increase the risk of group B streptococcal bacteriuria. This study revealed that group B streptococcal bacteriuria is more common in pregnancy than was previously suspected and suggests that culture methods to detect GBS should be used in bacteriuria screening programs done in pregnancy. In terms of perinatal infection risk, screening for group B streptococcal bacteriuria at or near the time of delivery may be more meaningful than other group B streptococcal surveillance culture studies.