In a seven-year follow-up study of a total county population, the incidence of coronary heart disease (CHD) was found to be lower in black than in white men in every occupational category except sharecroppers. Among nonfarming occupations the occupational gradient in CHD observed in the 1960 prevalence study was no longer present when incident cases were the basis for comparison. The lower prevalence rate in white farmers compared to nonfarmers, however, was confirmed in the incidence study. The most likely explanation for these findings was that sustained physical activity above a certain critical threshold value was protective against CHD. This explanation received some circumstantial support from the relationship discovered between leisure time and onthe-job physical activity and the prevalence of CHD. Hypotheses concerning the possible pathogenic processes through which physical activity might operate are briefly reviewed.