In a study of xanthomatosis kindreds in the county of Ostfold, Norway, 95 % of the living first degree relatives of the probands were investigated. Hyperlipoproteinaemia (lipid values above the 95th percentile) was present in 40.8 % of 554 first degree relatives of probands with xanthomatosis. The distribution curve was bimodal for cholesterol and LDL cholesterol concentrations, but not for triglyceride concentration within the different categories of families. The IIA lipoprotein pattern was the most frequent lipoprotein abnormality, in probands as well as in affected first degree relatives. However, IIB and IV lipoprotein patterns were also found in affected family members, irrespective of the pattern in the probands. About 93% of the xanthomatosis subjects had a lipoprotein disorder segregating as an autosomal dominant; the remaining 7 % were sporadic cases and/or had a multifactorially determined xanthomatosis. More sibs that offspring were affected; this was particularly pronounced for males with a IIA lipoprotein pattern. The genetic analysis gave no reason to suspect that hypercholesterolaemia with a IIA pattern is not the same disease as hypercholesterolaemia with a IIB pattern. However, a significant number of xanthomatosis patients had more than one type of hyperlipoproteinaemia. The frequency of the xanthomatosis trait was estimated to be 3.2/1000, and the ascertainment probability 0.6. The prevalence of familial hypercholesterolaemia with xanthomatosis was estimated to be 2.2/1000 and the multiple type hyperlipoproteinaemia with xanthomatosis had a frequency of 1.0/1000.