We assessed the role of parental history of premature coronary heart disease (CHD) in the risk of acute myocardial infarction (AMI) and the extent to which the risk associated with positive parental history was independent of other risk factors: smoking, high serum cholesterol, elevated blood pressure, diabetes and obesity, and of socioeconomic status. The study is a prospective 12-year follow-up of 15,620 men and women aged from 30 to 59 years in eastern Finland. Parental history of premature CHD was defined as either fatal or nonfatal myocardial infarction or angina pectoris before the age of 60 years. The end point of the follow-up was either nonfatal AMI or coronary death. The risk ratio (RR) of AMI associated with positive family history of either parent was 1.61 in men and 1.85 in women. The risk decreased only slightly when an adjustment was made for other risk factors, and did not change at all when an adjustment was made for the indicators of socioeconomic status. The risk was slightly higher for early AMI (< 55 years) compared with later AMI (> or = 55 years), RR 1.71 versus 1.50, among men and markedly higher, RR 2.87 versus 1.49, among women. These results from this population with an exceptionally high risk of CHD support the hypothesis that positive family history is an independent risk factor of AMI.