The purpose of this study was to determine the best scaling method to account for the effects of body size on measurements of overall cardiac function, and subsequently the interpretation of data based on cardiac power output (CPO). CPO was measured at rest and at maximal exercise on 88 and 103 healthy but untrained men and women over the age range of 20 to 70 years. Cardiac reserve (CR) was calculated as CPO_max - CPO_rest. CPO_rest, CPO_max and CR were all significantly related to body mass (BM), surface area (BSA) and lean body mass (LBM). The linear regression model failed to completely normalise these measurements. In contrast, the allometric model produced size-independent values of CPO. Furthermore, all the assumptions associated with the allometric model were achieved. For CPO_rest, mean body size exponents were BM^0.33, BSA^0.60 and LBM^0.47. For CPO_max, the exponents were BM^0.41, BSA^0.81 and LBM^0.71. For CR, mean body size exponents were BM^0.44, BSA^0.87 and LBM^0.79. LBM was identified (from the root-mean-squares errors of the separate regression models) as the best physiological variable (based on its high metabolic activity) to be scaled in the allometric model. Scaling of CPO to LBM^b dramatically reduced the between gender differences with only a 7% difference in resting and maximal CPO values. In addition the gender difference in CR was completely removed. To avoid erroneous interpretations and conclusions being made when comparing data between men and women of different ages, the allometric scaling of CPO to LBM^b would seem crucial.