Objective: To determine whether women with polycystic ovary syndrome (PCOS) are more likely to develop gestational diabetes mellitus compared with age- and weight-matched controls. Methods: This retrospective cohort study compared reproductive-age women with and without PCOS who received prenatal care at the University of North Carolina Hospitals between April 1989 and June 1998. We reviewed the medical charts of 22 women with PCOS diagnosis before pregnancy based on menstrual histories, elevated androgen levels, and LH-FSH ratios greater than 2. These women were compared with 66 women without PCOS matched for age and weight. Gestational diabetes mellitus (GDM) was diagnosed in women if they had abnormal results on a 50-g glucose screening test and at least two abnormal plasma glucose values during a 100-g glucose tolerance test. Medical complications of pregnancy, pregnancy complications, and birth outcomes were compared between women with and without PCOS. Results: Nine of 22 women with PCOS also had GDM diagnosis, compared with two of 66 controls (odds ratio [OR] 22.2; 95% confidence interval [CI] 3.8, 170.0), and these women exhibited increased plasma glucose values for all measurements except fasting. Five of 22 women with PCOS developed preeclampsia compared with one of 66 controls (OR 15.0; 95% CI 1.9, 121.5). Conclusion: Women with PCOS are at increased risk of glucose intolerance and preeclampsia during pregnancy.