A number of studies have reported hyperoestrogenaemia in men surviving an acute myocardial infarction (AMI). This has led to speculations that hyperoestrogenaemia might be a coronary risk factor. It is not clear why plasma levels of oestrogens should be elevated in coronary heart disease. To ascertain whether hyperoestrogenaemia might be present before the onset of a myocardial infarction, we performed a prospective study in 252 men above the age of forty who within the following 5 years suffered an AMI and, as controls, 526 men matched for age and coronary risk factors and 329 randomly selected men. Mean plasma concentrations of oestradiol and oestrone did not differ significantly between the groups. This suggests that hyperoestrogenaemia is not a coronary risk factor and thus, if present after the AMI, is more likely develop concurrently with the myocardial infarction.