Risk of first heart attack was found to be related Inversely to energy expenditure reported by 16,936 Harvard male alumni, aged 35–74 years, of whom 572 experienced heart attacks In 117,680 person-years of followup. Stairs climbed, blocks walked, strenuous sports played, and a composite physical activity Index all opposed risk. Men with Index below 2000 kilocalories per week were at 64% higher risk than classmates with higher Index. Adult exercise was Independent of other influences on heart attack risk, and peak exertion as strenuous sports play enhanced the effect of total energy expenditure. Notably, alumni physical activity supplanted student athleticism assessed in college 16–50 years earlier. If it Is postulated that varsity athlete status implies selective cardiovascular fitness, such selection alone Is insufficient to explain lower heart attack risk in later adult years. Ex-varsity athletes retained lower risk only If they maintained a high physical activity Index as alumni.