OBJECTIVE To evaluate the influence of maternal diabetes on the risk of group B streptococcus colonization during pregnancy. METHODS We prospectively analyzed data on 105 pregnant women with diabetes mellitus, both pregestational and gestational, and a control group of 300 pregnant women without carbohydrate intolerance. All had singleton gestations, negative tests for human immunodeficiency virus, and intact membranes at enrollment. Culture specimens for group B streptococcus were obtained from the lower vaginal walls and rectum. Two-tailed unpaired Student t test, Mann-Whitney U test, and chi 2 test were used as appropriate. Multiple logistic regression analyses were performed to evaluate the independent influence of maternal diabetes on the rate of group B streptococcus colonization. RESULTS Compared to controls, diabetic women had a higher colonization rate (43.8 versus 22.7%, odds ratio [OR] 2.56, 95% confidence interval [CI] 1.6, 4.1). The prevalence of group B streptococcus colonization in pregestational diabetic women was 54.1% and in women with gestational diabetes it was 35.1% (P = .05). Among women with pregestational diabetes, the prevalence of group B streptococcus colonization was 59.1% in class B and 50.0% in class C to R (P = not significant). After we adjusted for maternal age, race, and obesity, diabetic women continued to be at increased risk of group B streptococcus colonization (OR 3.1, 95% CI 1.8, 5.2). CONCLUSIONS Carbohydrate intolerance appears to be an independent risk factor for group B streptococcus colonization during pregnancy.