The objective of this study was to determine the prevalence of maternal group B Streptococcal (GBS) colonization and compare risk factor data related to GBS colonization. A prospective surveillance study of 598 pregnant women was conducted in two socioeconomically diverse maternity hospitals in Ribeirão Preto, Brazil between June and October 1999. Swabs from the lower vagina were obtained between 35 and 37 weeks gestation and cultured on selective media. Risk factor data were obtained by patient interview and chart review. The overall maternal GBS colonization prevalence rate was 17.9%. There was no association of GBS colonization with maternity hospital and no association of GBS colonization with previously identified risk factors, such as age, race, martial status, maternal education, parity, smoking, or alcohol use. There is a relatively high prevalence of maternal GBS colonization in this Brazilian population, although previously-identified-risk factors were not found to be important. This study provides baseline data for the creation of community-based GBS disease prevention protocols.