Genetic factors are known to be important in the etiology of pre-eclampsia and possibly also gestational hypertension, but the degree of genetic influence has not been quantified. To estimate the genetic and environmental effects on the liability of developing pre-eclampsia and gestational hypertension, we cross-linked the population-based Swedish Twin Register and the Swedish Medical Birth Register. We included female twin pairs with known zygosity, both of whom gave birth in Sweden from 1973 through 1993; in all 917 monozygotic and 1,199 dizygotic twin pairs. For pre-eclampsia, the estimates of heritability and nonshared environmental effect were 0. 54 (95% confidence interval 0-0.71) and 0.46 (0.29-0.67), respectively; corresponding estimates for gestational hypertension were 0.24 (0-0.53) and 0.76 (0.47-1.00), respectively. When considering both diseases as a single entity (pregnancy-induced hypertensive diseases), the heritability estimate was 0.47 (0.13-0. 61). These results suggest that genetic factors are important in the development of pre-eclampsia as well as gestational hypertension. The heritability estimates can be of importance when planning genetic linkage studies. In efforts to identify women with elevated risk of developing pre-eclampsia during pregnancy, a question about family history of pre-eclampsia should be included.