In comparing subjects of different body size echocardiographically measured data have usually been related to various body measures, mostly to body surface area (BSA). The present study serves to point out an inherent error of this procedure. Two-dimensionally guided M-mode echocardiography was performed in 106 elite male athletes. Left ventricular diastolic wall thickness (LVWT), internal diameter (LVID), and estimated left ventricular muscle mass (LVMM), stroke volume (SV) and cardiac output (CO) were related to body height (BH), body weight (BW), fat-free body mass (FFBM) and BSA. The analysis of the 20 indices has revealed that most indices and above all the BSA-related ones suffer from systematic errors so they are unsuitable to compare subjects independently from body size as they significantly correlate with BSA. These errors arise from the fact that the numerator and the denominator of these quotients differ in their power terms. With higher absolute component values all relativized quotients containing a numerator raised to a smaller power than its denominator is bound to decrease (e.g. LVWT/BSA, LVID/BSA) while quotients in which the numerator's exponent exceeds that of the denominator will increase (e.g. LVMM/BSA, CO/BSA). To avoid such spurious trends it would be wiser to use indices in which power terms match. When relations to BSA are preferred, its square root appears more suitable for linear numerators (wall thickness, diameters), while for volumes and weights the cube of the square root of BSA is suggested. In the general case, however, a dimensional consistence in choosing the relativizing parameter is recommended.