This study examined the effects of calorie alternation and exercise on weight loss. Moderately obese women (130-160% of ideal body weight) were randomly assigned to an alternating- or constant-calorie diet with or without aerobic exercise. Both diets provided an average of 1200 kcal/d over a 12-wk period; daily intake of subjects in the alternating-diet condition varied in a prescribed pattern from 600 to 1800 kcal/d. Exercising subjects walked 5 d/wk. All subjects participated in an intensive outpatient behavior-modification program. At the end of the study, exercised subjects had greater reductions in body weight and body fat percentage than did nonexercised subjects. The type of caloric restriction did not affect weight or fat loss. Changes in resting metabolic rate did not differ among groups. Alternating calories was neither beneficial nor detrimental as a weight-loss strategy whereas exercise was clearly beneficial in weight-loss therapy.