Resting metabolic rate (RMR) in adults can be viewed from two perspectives. Body surface area is used to index RMR in the clinical perspective. Normal values of RMR are thus sex-specific and appear to be lower in women. However, the physiological perspective is one of an asexual metabolism that differs in the mass of active cells. The body cell mass is relatively reduced in women due to a smaller skeletal muscle mass in tandem with a greater fat mass. RMR is proportionately reduced and thus highly correlated with body cell mass or lean body mass in mixed sex data. Athletic training can negate the differences in body composition and RMR per unit weight. The body mass index is another nonphysiological and sex-specific parameter currently being applied to obesity research. The clinical utility of body surface area for the prediction of RMR results from its correlation with body composition within either sex. The clinical and physiological perspectives of RMR must be clearly distinguished and treated as distinct.