Gestational diabetes and pregnancy-induced hypertension are common, and their relation is not well understood. The authors conducted a population-based case-control study using 1992-1998 Washington State birth certificate and hospital discharge records to investigate this relation. Consecutive cases of pregnancy-induced hypertension were divided into four groups based on International Classification of Diseases, Ninth Revision codes: eclampsia (n=154), severe preeclampsia (n=1,180), mild preeclampsia (n=5,468), and gestational hypertension (n=8,943). Cases were compared with controls who did not have pregnancy-induced hypertension (n=47,237). Gestational diabetes was more common in each case group (3.9% in eclamptics, 4.5% in severe preeclamptics, and 4.4% in both mild preeclamptics and those with gestational hypertension) than in controls (2.7%). After adjustment for body mass index, age, ethnicity, parity, and prenatal care, gestational diabetes was associated with increased risk of severe preeclampsia (odds ratio (OR)=1.5, 95% confidence interval (CI): 1.1, 2.1), mild preeclampsia (OR=1.5, 95% CI: 1.3, 1.8), and gestational hypertension (OR=1.4, 95% CI: 1.2, 1.6). Gestational diabetes was more strongly associated with pregnancy-induced hypertension among women who received less prenatal care (OR=4.2 for eclampsia and OR=3.1 for severe preeclampsia, p<0.05 for both) and among Black women (OR for eclampsia and preeclampsia together=3.9, p<0.05).