There is an increased interest in understanding how variation in body composition (BC) and energy expenditure is related to successful weight loss after surgery. It has been suggested that low resting metabolic rate (RMR) could be associated with poor weight loss. To determine the relation among changes in BC, RMR, and weight loss after bariatric surgery. University tertiary care hospital, Brazil. A cohort of 45 patients submitted to bariatric surgery was prospectively studied. BC was evaluated by bioelectrical impedance analysis and RMR by indirect calorimetry before and 6 months after surgery. The RMR value was adjusted per kilogram of weight (RMR/kg). The patients were divided in 4 groups, based on patterns of change in the RMR/kg after surgery. The RMR/kg could decrease (group 1), remain stable (group 2), have a small increase (group 3), or have a major increase (group 4). A significant relation between fat-free mass and RMR for both pre- (P<.01) and postoperative periods (P<.01) was observed. Excess weight loss had a significantly correlation only with post-RMR/kg (P<.01). The pattern of change in RMR/kg was strongly correlated with weight loss, considering an excess weight loss>50% a successful weight loss: No patients achieved success in group 1; 61% of patients did in group 2; 80% di in group 3; and all patients in group 4 had successful weight loss. We demonstrate a clearly correlation between the postoperative RMR and weight loss. The increase in RMR/kg after surgery is a major factor related to a satisfactory excess weight loss after surgery.