Twenty-three probands with idiopathic hemochromatosis were assigned the status of homozygotes: 132 of their relatives were classified as homozygotes, heterozygotes or normal individuals using the HLA haplotypes of the probands as markers of the hemochromatosis allele. Only half of the probands sought help because of symptoms or signs of iron overload. Clinical manifestations of iron loading were present, however, 95% of the probands and 67% of the discovered homozygotes. The commonest symptom was joint pain and stiffness. None of the heterozygotes had any clinical symptoms of excess body iron. High transferrin saturation and serum ferritin levels were prevalent in homozygotes: only 1 of 38 homozygotes had values for both of these measurements that were within normal limits. The level of transferrin saturation was increased in 6% of heterozygotes but only 1% had serum ferritin concns greater than 350 ng ml-1. The mean radioiron absorption levels of 27 homozygotes and 28 heterozygotes were similar to those in 44 controls. Radioiron absorption in relation to the respective serum ferritin concn was above the 95% confidence interval of controls in 65% of the homozygotes and 7% of the heterozygotes. The inverse relation between radioiron absorption and the respective serum ferritin concn observed in controls was absent in homozygotes but remained strong in the heterozygotes. Absence of the inverse relation indicates a deregulation of the iron absorptive mechanism in homozygotes which results in the size of body iron stores having no effect on the level of iron absorption.(ABSTRACT TRUNCATED AT 250 WORDS)