In a study of xanthomatosis in the county of Ostfold, Norway (approximately 220,000 inhabitants), it was found that 20% (37 out of 185) of the probands belonges to seven large kindreds where xanthomatosis and hyper-beta-lipoproteinaemia segregated. Almost complete ascertainment (99%) of living subjects in the sibships in these families was obtained. Within each kindred, the frequency distribution of age- and sex-adjusted cholesterol and LDL cholesterol were discontinuous; the xanthomatosis patients all had values corresponding to the upper mode, whereas no person without xanthomatosis had such values. The combined results showed a slight overlap. Triglyceride concentrations were unimodally distributed. The genetics of the disorder in the seven families was analysed on the basis of 270 individuals. The segregation pattern satisfied the criteria for autosomal dominant inheritance, but not those for a polygenic trait. Thus, xanthomatosis with hyper-beta-lipoproteinaemia segregates as an autosomal dominant trait in these seven kindreds. No evidence of reduced penetrance was found. Since both IIA and IIB lipoprotein patterns were observed within the same family, there is no evidence that the two patterns reflect the presence of different genes in these kindreds.