Summary:To determine the recurrence risk of West syndrome (WS), we studied the familial antecedents of consecutively referred patients. Among siblings, there was an increased incidence of WS but not of febrile convulsions. Familial incidence of epilepsy was intermediate between the epileptic and nonepileptic control groups. When cases resulting from a genetically determined disease were excluded, incidence of epilepsy among siblings was similar to that in normal controls. Five of the 11 familial cases of WS were due to an identifiable cause: twin pregnancy, tuberous sclerosis, and recurrent maternal toxemia. In 4 of the remaining families, the clinical picture included spasms, erratic myoclonus, and postnatal microcephaly, suggestive of a previously unidentifiable progressive encephalopathy. Therefore, when identifiable familial diseases were excluded, the recurrence risk was <1%.