BACKGROUND Bariatric surgery has become the method of choice for weight loss in severely obese patients. While the incidence of obesity and its co-morbidities have been well-documented, less is known about the outcomes of the surgery as it relates to body composition and metabolic changes. METHODS 19 bariatric surgery patients (14 female, 5 male) were assessed for basal metabolic rate (BMR), percent fat, fat mass, and lean body mass (LBM) just prior to surgery and at 1, 3 and 6 months after surgery. RESULTS Analysis of Variance (ANOVA) using the general linear model indicated significant changes in all measures of body composition for each time period. Linear regression was developed for each of these measures. Of the 39.7-kg weight loss, 26.4 kg (66.5%) was fat mass loss and 13.3 kg (33.5%) was LBM loss. BMR significantly decreased from pre-surgery (2091 kcals) to 1 month post-surgery (1758 kcals), with no significant changes in subsequent time periods. Repeated measures ANOVA for the ratio of BMR/LBM indicated no significant differences for any comparison of time periods (F=2.29, P=.111), suggesting that there is no adaptation of an energy-conserving mechanism in these patients. CONCLUSION Bariatric surgery was highly successful in inducing significant changes in body weight (39.7 kg), fat mass (26.4 kg), % body fat (7.9%) but also lean body mass (13.3 kg). The 440 kcal decrease in BMR, while significant, was not greater than expected due to significant decreases in LBM.