Previous studies suggest that risk of premature myocardial infarction is increased by a heritable factor not associated with the conventional risk factors. Among the 5-7% of persons in the general population at high risk, the men are at greater risk than the women. It is proposed that heterozygosity for hemochromatosis increases the susceptibility of the carrier to premature myocardial infarction. This hypothesis explains much of the behavior of the unknown heritable factor, including its independence from the conventional risk factors and its apparent selectivity for men under age 50. Heterozygous hemochromatosis occurs frequently enough to account for the observed number of high risk families. Studies for verification of the hypothesis are proposed.