In order to distinguish between a maternal, fetal or maternal and fetal genetic predisposition towards severe pre-eclampsia, the first pregnancies of 158 mothers and 160 mothers-in-law of pre-eclamptic women and of matched controls were analysed. Fourteen per cent of mothers of pre-eclamptics were found to have had severe pre-eclampsia, confirming previous suggestions that the condition "runs in families', in contrast to only a 3% incidence amongst mothers of controls. The incidence in mothers-in-law of both pre-eclamptics and controls was 4%, in full agreement with a maternal genotype hypothesis and suggesting that the fetal genotype plays, at most, only a minor role in the aetiology of severe pre-eclampsia. The data are in agreement with the hypothesis that a single recessive gene acting in the mother could be responsible for severe pre-eclampsia, but multifactorial inheritance is not ruled out. Mild pre-eclampsia showed no such familial tendency, indicating that the mild and severe forms of pre-eclampsia may represent separate pathological entities.