The Harvard Alumni study is reviewed critically with a view to resolving the apparent conflict between the conclusions drawn from this research and the current consensus on optimal patterns of physical activity for health. The optimal energy expenditure reported for the Harvard data set is less than at first appears, since the estimated expenditures are gross rather than net values. Further, the optimal energy expenditure has been over-estimated, because too high a cost was assumed for stair climbing, and sport involvement may also have been over-reported. The data do not as yet allow the assertion that benefit is obtained only from vigorous physical activity, and there seems little conflict with the current consensus on the benefits of moderate physical activity.