Objective: This study was planned to investigate the effects of diet and physical activity on resting metabolic rate (RMR) measured by indirect calorimetry, and body composition assessed by dual energy-X-ray absorptiometry (DXA).Materials and Methods: This is a longitudinal, clinical intervention study of weight loss diet daily with/without exercise for 12 weeks. Overweight women with a body mass index (BMI): 25.0-29.9 kg/m2 and obese women with a BMI>30.0 kg/m2 (n:37), aged 20-45 years were included in the study. The subjects were divided into two groups: DA - the group received diet alone (n:20), DPA - the group received diet and exercise therapy (n:17). Various anthropometric measurements were performed; body composition of the subjects were measured by DXA and bioelectrical impedance analyzer (BIA) and resting energy expenditure (REE) was assessed by Cosmed K4 B2 at the beginning and end of the study. Results: Basal metabolic rate (BMR) and RMR during the weight loss program in DA group were significantly lower than at baseline (p<0.001). While BMR measurements decreased, RMR levels increased significantly in DPA group at the end of the study (p<0.001). There were significant differences between the groups in terms of body weight (kg) (p=0.001), body fat mass (kg) (p=0.001) and body fat percentage (%) (p<0.05) that was measured by DXA. Conclusion: Diet and exercise together could accelerate body fat loss, preserve fat-free mass and prevent/increase in RMR more effectively than with diet restriction alone. It is suggested that, in order to ensure healthy weight loss, increasing physical activity in addition to diet should be recommended. Turk J Phys Med Re-hab 2012;58:1-8.