The effects of caloric restriction and exercise on body composition, resting metabolic rate (RMR), and maximal oxygen uptake (VO2 max) were studied for 16 wk in 26 premenopausal obese women. Exercise (X) vs nonexercise (NX) was crossed with a low-fat, ad libitum-carbohydrate (AL) diet vs a restricted (R) (800 kcal) low-fat, high-carbohydrate diet in a 2 x 2 factorial design. Subjects were randomly assigned to one of the four treatment groups. Body-weight and percent-fat losses were significant (p less than 0.05) in all groups but greater in subjects assigned to the R diet (p less than 0.05) and/or X (p less than 0.10) groups. Exercise increased (p less than 0.01) VO2 max but neither exercise nor diet influenced fat-free mass or RMR (kcal.m-2.h-1), both of which remained unchanged over time. A program similar to that followed by the ALX group is recommended for long-term weight control and overall health.