This study assessed the effects of 16 weeks of energy restriction and vigorous exercise on body mass and body composition. Sixty sedentary men, mean body mass (mean +/- SD) 96.3 (13.9) kg and mean age 42.4 (5.0) years, were randomly assigned to either continue their normal energy intake or restrict energy intake by 4,186 to 6,279 kJ. d(-1). Each group was further randomized to a control light exercise program, or a vigorous exercise program for 3 half-hour sessions per week. Vigorous exercise improved maximum oxygen consumption (Vo(2max)) by approximately 24% (0.56 [95% confidence interval, 0.47 to 0.65) L. min(-1), P <.001) with no significant changes in body mass, body composition, or fat distribution. With energy restriction there was a significant reduction in body mass of 10.1 (8.0 to 12.2) kg, lean body mass (LBM) of 2.4 (1.5 to 3.3) kg, fat mass (FM) of 7.7 (5.9 to 9.6) kg, waist to hip ratio (WHR) of 0.03 (0.01 to 0.04), and the sum of 6 skinfolds of 26.9 (15.4 to 38.4) mm (P <.001). Combining vigorous exercise with energy restriction resulted in no further changes in measures of body composition. We conclude that in sedentary free-living overweight men, 16 weeks of energy restriction, but not vigorous intensity exercise, results in substantial reductions in body mass, LBM, and FM. Furthermore, vigorous intensity exercise when combined with energy restriction did not modify or enhance the changes in body fat distribution or body composition seen with energy restriction alone. The independent effects of exercise to induce changes in body mass and composition in the longer term in free-living overweight subjects on an energy-restricted diet deserve further study. Copyright 2003, Elsevier Science (USA). All rights reserved.