Objective To examine the association between ABO and RhD blood groups and gestational hypertensive disorders in a large population-based cohort.Design Cohort study. Risks of gestational hypertensive disorders, pre-eclampsia, and severe pre-eclampsia, estimated by odds ratios for maternal ABO blood group and RhD status.Setting National health registers of Sweden.Population All singleton deliveries in Sweden born to first-time mothers during the period 1987-2002 [total n = 641 926; any gestational hypertensive disorders, n = 39 011 (6.1%); preeclampsia cases, n = 29 337 (4.6%); severe pre-eclampsia cases, n = 8477 (1.3%)].Methods Using blood group O as a reference, odds ratios of gestational hypertensive disorders, pre-eclampsia, and severe preeclampsia were obtained from logistic regression models adjusted for potential confounding factors.Main outcome measures Gestational hypertensive disorders, preeclampsia, and severe pre-eclampsia.Results Compared with blood group O, all non-O blood groups had modest but statistically significantly higher odds of preeclampsia. Blood group AB had the highest risk for pre-eclampsia (OR = 1.10, 95% CI 1.04-1.16) and severe pre-eclampsia (OR = 1.18, 95% CI 1.07-1.30). RhD-positive mothers had a small increased risk for pre-eclampsia (OR = 1.07, 95% CI 1.03-1.10).Conclusions In the largest study on this topic to date, women with AB blood group have the highest risks of gestational hypertensive disorders, pre-eclampsia, and severe pre-eclampsia, whereas women with O blood group have the lowest risks of developing these disorders. Although the magnitude of increased risk is small, this finding may help improve our understanding of the etiology of pre-eclampsia.