The relation between xanthomatosis and plasma lipid and lipoprotein concentrations has been studied in 336 subjects (172 males and 164 females) with xanthomatosis. With a few exceptions, these patients were either born or lived in Ostfold county, which has a population of about 220 000. Xanthomas had made their first appearance in half the subjects by the age of 48 years. The most frequent localizations of xanthomas were the extensor finger and the Achilles tendons, the former site being more common in males and the latter in females. Tuberotendinous xanthomas were found in 7% of the cases, whereas only one subject had tuberous xanthomas without tendinous involvement. Xanthelasmas were found in 1/3 of the females and 1/6 of the males. The size of xanthomas and the number of xanthoma sites increased with age and cholesterol levels. Cholesterol concentrations were (or had been) increased in 5/6 of the subjects with xanthomathosis. Normal cholesterol was combined with increased triglyceride values in 5% of the individuals. A further 6% had normal cholesterol and triglyceride values together with raised LDL cholesterol concentrations, whereas the remaining subjects had normal lipid levels. There was no evidence of homozygosity for the gene(s) for type II hyperlipoproteinaemia. The type III hyperlipoproteinaemia pattern was not seen. Type IIA and IIB electrophoretic patterns were seen in 68 and 13% of those tested, respectively. Changes in lipoprotein patterns with treatment were frequent, especially in males, and the "original" frequency of type IIA is probably higher.