Arthur J. Siegel, MD, regarding the relative risk of cardiac death during marathon running, raises the interesting question of how much exercise is too much. Unfortunately, the data are not yet available to answer this question. What data are available suggest that progressively increasing exercise levels progressively decrease the risk for coronary artery disease. This appears true despite the fact that exercise transiently increases the risk for both sudden cardiac death and myocardial infarction as summarized in my review' and in Dr Siegel's letter. At the present time, however, too much exercise is not a public health problem. More Americans die of too little physical exertion rather than of too much.