The purpose of this study was to determine the optimal index for normalizing left ventricular (LV) echocardiographic dimensions for differences in body size. M-mode echocardiograms defined LV internal dimension at end diastole (LVIDD) and LV wall thickness (LVWT) in 107 adults (59 male, 48 female). Allometric relations were assessed between cardiac dimensions (Y) and body size variables (X) of fat-free mass (FFM), height (H), body surface area (BSA), and fat mass (FM). Further to confirmation of homogeneity of regression slopes, size exponents common to both genders were fitted by a log-linear model: ln Y = ln a + c.gender + b.ln X, where a is the proportionality coefficient, b is the size exponent, and c is the gender coefficient. For LVIDD, mean body size exponents (95% confidence interval) were FFM0.35 (0.22-0.47), H0.68 (0.32-1.03), and BSA0.44 (0.26-0.62). For LVWT, the derived exponents were FFM0.43 (0.20-0.65), H0.65 (0-1.3), and BSA0.56 (0.23-0.89). Body fatness (expressed by FM) had no influence on LV dimensions, with exponents not different from zero (P > 0.05). The root-mean-squares error from the separate regression models indicated that the FFM index was the optimal solution. Indexation of LV dimensions by H was associated with the greatest error. Because the 95% confidence interval for the FFM exponents included 0.33, we recommend that linear LV dimensions be indexed by the cube root of FFM. In the absence of FFM data, the root of BSA was found to be the best surrogate index.