A genetic study of 75 cases of bovine spongiform encephalopathy (BSE) of which 51 were confirmed by histopathology in 29 pedigree and seven non-pedigree herds of Holstein Friesian cattle revealed that 73 per cent of 60 BSE cases had first or second degree relatives also affected. All the 44 cases assigned to families could be traced back in the previous three generations to one cow and 11 bulls, which were of Canadian Holstein or Dutch Friesian heredity. No single common ancestor could be identified in the parentage of BSE-affected animals in pedigree studies up to six or more generations. The number of common ancestors and the degree of relatedness of the affected animals in a multiple-case herd was no more than would be expected from the breeding structure of the herd. The segregation ratio of affected cows in the proband generation within sire and maternal grandsire sibships in 12 pedigree herds was not inconsistent with Mendelian expectation for autosomal recessive inheritance with complete penetrance. The data analysed shows that the disease itself is not simply inherited. However, there remains a real possibility that the susceptibility of individual animals to BSE is inherited. This should be taken into account in current and future research on the aetiology and control of the disease.