C282Y homozygosity is necessary (but insufficient in isolation) for the onset of hemochromatosis, as indicated by its low biochemical penetrance (75 % in men and 50 % in women) and clinical penetrance (25 % in men and 0 % in women). Factors modulating iron load may be acquired (diet, alcohol, metabolic syndrome, drugs, etc.) or, more importantly, genetic (digenism, polymorphism of genes involved in the regulation of hepcidin synthesis). Factors modulating iron-related organ damage include alcohol consumption, the metabolic syndrome, and the TGF-beta1 (hepatic fibrosis) and superoxide dismutase genes (cardiomyopathy). Further studies of these modifiers are needed to improve the management of C282Y homozygotes, at both the individual and the population levels.