In a study of 20 families with idiopathic hemochromatosis, relatives of probands were classified as either homozygous, heterozygous, or normal according to their HLA phenotype. An abnormality in the serum iron concentration, total iron-binding capacity, or serum ferritin concentration was present in all homozygotes and in 25% of heterozygotes. In heterozygotes, the mean total iron-binding capacity was significantly decreased, and the mean hepatic iron concentration was significantly increased compared to normals. However, in contrast to homozygotes, clinical evidence of iron overload was not observed in heterozygotes, and there was no biochemical or histological evidence of liver disease resulting from excessive iron stores. Progressive iron overload did not develop in 44 heterozygotes who were studied for up to 16 yr.