Hereditary hemochromatosis is an autosomal recessive disease in which the gene is linked to the HLA system. Investigation of nine unrelated probands and their family members has revealed distinct groups based on biochemical and clinical manifestations of the disease. Four different types of disease expression were identified: Group I--classic hereditary hemochromatosis with elevated transferrin saturation, serum ferritin levels, and liver iron content; Group II--severe iron overload, accelerated disease manifesting at an early age; Group III--elevated total body iron stores, normal transferrin saturation and serum ferritin levels; Group IV--markedly elevated findings on serum biochemical tests, e.g., transferrin saturation, serum ferritin levels, with minimal elevation in total body iron stores. This evidence for several clearly distinguishable modes of expression in different families suggests that more than one genetic lesion in iron metabolism may be responsible for iron overload in hereditary hemochromatosis. This genetic heterogeneity may be helpful in delineating the fundamental abnormalities in iron metabolism in this group of disorders.