Sixty-six idiopathic hemochromatotic French patients were HLA-A, B typed. The previously known strong association with A3 was confirmed (RR = 10.6, P less 10(-9)) and our results indicate clearly that a gene implicated in idiopathic hemochromatosis (IH) determinism is located in the HLA-A region. The linkage disequilibrium between A3 and B7 was found to be far greater in IH patients than in controls. The authors have therefore hypothesized that this might be due to a selective advantage of this haplotype in IH. The A3, B7, Dw2 HLA haplotype has been shown to exert a protective effect against common insulin dependent diabetes (IDD). Thus the patients were divided into two groups according to the presence or absence of a definite IDD. B7 was found more frequently in IH patients without IDD but the difference is not significant. In this context, the strong linkage disequilibrium between A3 and B7 might be due to the protective effect of the B region of this haplotype against IH secondary IDD.