Gestational hypertension (GHTN) and gestational diabetes mellitus (GDM) are both insulin resistance states. Perinatal outcome of GHTN or GDM alone are well established, but their combined effect on pregnancy outcome is underinvestigated. Our objective was to determine if pregnancies complicated by GHTN/GDM have higher rates of morbidity. We identified nulliparous women with singleton pregnancies delivering at 37 to 40 weeks of gestation from 1995 to 2004 from a database. Outcomes of pregnancies complicated by GHTN only, GDM only, or combined GHTN/GDM were compared with controls. Data analysis included the Mann-Whitney U test, the Kruskal-Wallis H test, and analysis of variance. Multivariate analysis was used to adjust for confounders. Of 14,880 patients, there were 11,349 controls, 2604 GHTN, 728 GDM, and 199 GHTN/GDM. After controlling for covariates, GHTN significantly increased cesarean section (C/S) rate (odd ratio [OR], 1.62; confidence interval [CI], 1.47 to 1.78), rates of admittance to the neonatal intensive care unit (NICU), and birth of large for gestational age (LGA) infants. GDM significantly increased C/S (OR, 1.42; CI 1.21 to 1.66), rates of NICU admission (OR, 1.32; CI, 1 to 1.75), birth of LGA (OR, 1.51; CI 1.14 to 1.98), and macrosomic infants (OR, 1.53; CI, 1.12 to 2.08). Rates of LGA infants (OR, 1.85; CI, 1.19 to 2.86) and C/S (OR, 2.03; CI, 1.52 to 2.71) were significantly increased with GHTN/GDM. We concluded that GHTN or GDM is associated with increased rates of adverse outcomes. Their coexistence further increases adverse perinatal outcomes.