High frequencies of the haemochromatosis-related HFE C282Y mutation have been reported in North European populations, in which a high proportion of patients with the disease are homozygotes. However, the degree of penetrance of this genotype is unknown. We determined the HFE C282Y and H63D genotypes of 411 consenting volunteer blood donors on Jersey, and the serum ferritin and transferrin saturation levels of 204 of these volunteers. The C282Y allele frequency was found to be 8.3% in 822 chromosomes, indicating a homozygote frequency of 1/145. Consistent with this, four C282Y homozygotes were detected in 411 volunteers. As there are only 18 patients presently receiving treatment for haemochromatosis on Jersey, out of a total population of about 85000, there is a large discrepancy between the number of haemochromatosis patients and the number of C282Y homozygotes in this population. In a preliminary study of 204 consenting volunteers we found a correlation between transferrin saturation and HFE H63D/ C282Y genotype (P=0.017) and between serum ferritin and genotype (P = 0.056). We also observed elevated values of transferrin saturation in the two C282Y homozygotes assayed. These results suggest that a large proportion of the many undetected C282Y homozygotes on Jersey and in similar populations could be in the preclinical stages of haemochromatosis, and warrant investigation. However, there may be a wide variation in the expression of the condition, and a more extensive study of the level of disease penetrance encompassing a large number of hitherto undetected C282Y homozygotes is therefore imperative.