Iron deficiency is the most common disorder of iron metabolism worldwide, but there is concern that iron accumulation resulting from enhanced iron absorption may also be a cause of morbidity. In patients with genetic haemochromatosis the clinical manifestations of iron overload are well-known. In northern Europe 90% of such patients are homozygous for the C282Y mutation of the HFE gene and this genotype is found in 1 in 200 of the population. Heterozygosity for C282Y occurs in 15% of the population and 25% carry another mutation, H63D. Population studies have revealed (i) the serum transferrin saturation is strongly influenced by HFE genotype, being lowest in subjects lacking mutations and highest in those homozygous for C282Y; (ii) most subjects homozygous for C282Y accumulate iron but do not present with the clinical manifestations of iron overload. Testing for HFE mutations in clinics for diabetes, liver disease and cardiovascular disease has shown that homozygosity for C282Y is not commonly found. Heterozygosity for either C282Y or H63D does not appear to be a risk factor for these common conditions.