Twelve biological and lifestyle characteristics measured in a group of 5919 middle aged men free of clinical coronary heart disease (CHD), stroke and cancer were analyzed for differences in predicting early and late onset of new cases of definite CHD (non-fatal myocardial infarction and fatal CHD) over a 12-year follow-up period. Among these men, 151 cases of definite CHD occurred early (under age 60) and 135 cases occurred later in life (age 60 and over). Serum triglyceride was the only risk factor that was an independent predictor of early onset disease and not of late onset disease. While cigarette smoking was a predictor for both onset groups, the effect of smoking was greater in people who developed CHD earlier in life. Systolic blood pressure, alcohol intake, serum cholesterol and serum glucose were independent predictors for both onset groups, with no difference in effect between groups. The findings indicate that it is possible for some factors to affect CHD risk differently in terms of premature vs delayed onset of disease. The findings for serum triglyceride may account for some of the inconsistencies in reports regarding it as an independent risk factor for CHD. In general, however, most of the characteristics studied here had a similar effect on both early and late onset and thus remain important in the prevention of both premature and late onset of CHD.