Occupational and leisure time physical activity and conventional risk factors were determined in the Belgian Physical Fitness Study, a prospective study of 2,383 healthy male factory workers who were aged 40–55 years at entry in 1976–1978 and who were followed for five years. Physical fitness, defined as the interpolated physical working capacity at heart rate 150 beats per minute, was measured in 2,109 subjects. In this subgroup, there were 31 myocardial infarctions and sudden deaths. Smoking, physical fitness, and high density lipoprotein cholesterol (HDL cholesterol) were independent risk indicators for subsequent ischemic heart disease, while both physical activity scores were not. It is concluded that in this healthy, predominantly sedentary population, the fitness level, but not the physical activity pattern, is an independent protective factor against ischemic heart disease.