The objective of this study was to evaluate and compare risk factor patterns in association with preeclampsia and gestational hypertension. The data were collected from The Swedish Medical Birth Register and include all nulliparas aged 34 years or less who gave birth at the University Hospital of Uppsala, Sweden, during 1987-1993. Of these 10,666 women, 4.4% developed gestational hypertension, and 5.2% developed preeclampsia. The following risk factors were significantly associated with increased risk of preeclampsia: type 1 diabetes (odds ratio = 5.58, 95% confidence interval 2.72-11.43), gestational diabetes (odds ratio = 3.11, 95% confidence interval 1.61-6.00), and twin birth (odds ratio = 4.17, 95% confidence interval 2.30-7.55). The positive associations between these variables and the risk of gestational hypertension were weaker and nonsignificant. Compared with underweight women (body mass index < 19.8), obese women (body mass index > 29) had increased risks of both gestational hypertension (odds ratio = 4.85, 95% confidence interval 1.97-11.92) and preeclampsia (odds ratio = 5.19, 95% confidence interval 2.35-11.48). Significantly lower risks of preeclampsia and gestational hypertension were observed for women born outside Nordic countries and in association with maternal smoking and summer birth. The similarities in risk factor patterns may indicate similarities in the biologic mechanisms underlying the two conditions.