Epidemiological studies have failed to assess the extreme exercise loads and cardiac adaptation characteristic of highly trained endurance athletes. Data are presented from a study assessing mortality according to fitness levels in men (open bars) and women (closed bars) demonstrating an “asymptote of benefit” at a V˙O2max of approximately 35 mL·min−1·kg−1 (adapted from Blair SN, Kohl HW 3rd, Paffenbarger RS Jr, et al. Physical fitness and all-cause mortality. A prospective study of healthy men and women. JAMA. 1989; 262: 2395–401). Representative echocardiograms are superimposed. A 23-yr-old accountant with normal cardiac size and a V˙O2max of 33 mL·min−1·kg−1 is compared with a 23-yr-old elite cyclist with a V˙O2max of 81 mL·min−1·kg−1 and a heart that is more than twice as large. The data derived from populations in which exercise-induced cardiac changes are minimal cannot be extrapolated to include populations with an entirely different exercise-induced phenotype.