An ongoing family study of idiopathic hemochromatosis in Brittany, France, allowed us to investigate the segregation of this trait and its linkage and association to the HLA-A locus in 147 pedigrees, comprising 1,408 individuals with over 900 characterized for relevant biological parameters and typed for HLA. The joint consideration of affection status and serum iron concentration reveals no dominance effect on the latter trait and documents the increased information afforded by the consideration of a biological correlate of liability to affection for disease exhibiting incomplete penetrance. Our overall results are in general agreement with published results on a Utah family study.