Familial predisposition and patterns of genetic inheritance of eclampsia and pre-eclampsia were investigated through three or four generations in 94 families from the homogenous island population of Iceland. The families descended from index women delivered in the years 1931-47 and who had either eclampsia (n = 38) or severe pre-eclampsia (n = 69). Inheritance was followed both through sons and daughters. The prevalence of pre-eclampsia and eclampsia in daughters was significantly higher (23%) than that in daughters-in-law (10%). No difference was noted in the prevalence of these diseases by whether the daughter was born of an eclamptic or pre-eclamptic mother or whether she was a first or later born daughter. There was a non-significantly higher occurrence of pre-eclampsia among grand-daughters than in grand-daughters-in-law. No difference was seen by whether grand-daughters descended through sons or daughters. With increasing numbers of affected daughters or grand-daughters the probability rose of finding more affected women in a family. Hypotheses of single recessive and dominant gene inheritance were compared and maximum likelihood estimates for gene frequency obtained. For a single recessive gene model this was 0.31 reflecting a population prevalence of 9.6%, whereas a dominant model with incomplete penetrance gave 0.14 at 48% gene penetrance, corresponding to a population prevalence of 0.9% homozygous expression of severe disease and 11% heterozygous expression of milder disease. Either genetic model could fit the data.