A candidate gene for hemochromatosis has recently been localized on the short arm of chromosome 6, about 4 megabases telomeric to the major histocompatibility complex. It encodes a protein that exhibits significant similarity to the HLA class I molecules and can be provisionally designated HLA-hc. Genotype analysis of 94 hemochromatosis patients living in France and a similar number of controls confirms that the disease is strongly associated with homozygosity at nucleotide 845 (72% of the patients and none of the controls carry two copies of the 845A variant). The data are consistent with hemochromatosis being a heterogeneous disease: about 79% of the cases in this sample would be caused by a defect in HLA-hc and 21% by an unrelated mechanism. A second variant (187 G) enriched on patient chromosomes that do not carry the 845A mutation might influence the affinity of a ligand for HLA-hc; the exact nature of this ligand remains to be discovered. The 845A variant is the best genetic marker for the disease identified to date, and the detection of 845A homozygosity should now permit diagnosis of a readily curable disease and the prevention of sometimes deadly complications in at least 72% of the patients.