OBJECTIVE: While women with prior gestational diabetes mellitus (GDM) are more likely to display features of the metabolic syndrome, including hypertension, in the years after delivery, it is unclear whether these components are also present before pregnancy. We examined the relationship between blood pressure (BP) measured before and during early pregnancy (<20 weeks) and the risk of GDM in a nested case-control study. RESEARCH DESIGN AND METHODS: Case (n = 381) and control (n = 942) subjects were selected from a cohort of women delivering between 1996 and 1998 and screened for GDM between 24 and 28 weeks' gestation. GDM was defined by the National Diabetes Data Group criteria. BP and covariates data were obtained by review of the medical records. Women were categorized according to BP levels recommended by the American Heart Association outside of pregnancy: <120/80 mmHg (normal), 120-139/80-89 mmHg (prehypertension), and >or=140 and/or >or=90 mmHg or use of antihypertensive medications (hypertension). RESULTS: During early pregnancy, women with prehypertension had a small increased risk of GDM (odds ratio [OR] 1.56 [95% CI 1.16-2.10]), and women with hypertension had a twofold increased risk of GDM (2.04 [1.14-3.65]) compared with women with normal BP after adjusting for age, race/ethnicity, gestational week of BP, BMI, and parity. Similar results were seen among the subset of women with BP levels measured before pregnancy (1.44 [0.95-2.19] for prehypertension and 2.01 [1.01-3.99] for hypertension). CONCLUSIONS: Clinicians should be aware that women presenting with hypertension may warrant early screening or intervention to prevent GDM.