Twenty-six cases of endocardial fibroelastosis were collected from three hospitals in Manchester over a ten-year period. Nine cases occurred in 4 families and these are discussed in detail. X-linked recessive inheritance seems likely in one family in which two probable female carriers had subarachnoid haemorrhages. In a second family an apparently normal man produced two children with endocardial fibroelastosis by different mothers suggesting autosomal dominant inheritance with incomplete penetrance. Autosomal recessive inheritance may be involved in the remaining two families but this was not associated with consanguinity. Genetic heterogeneity is evident in endocardial fibroelastosis and the majority of cases occur sporadically. An accurate family history is therefore necessary but it is difficult to give precise recurrence risks in sporadic cases.