Recent evidence suggests that dietary fat intake may be more strongly associated than total energy intake to the development and maintenance of obesity. The objective of this study was to determine whether dietary fat restriction would promote more favorable changes in body weight, body composition, resting metabolic rate (RMR), eating behavior, and dietary adherence than calorie restriction. Sixty-five obese women and 15 obese men were recruited and randomly assigned to calorie restriction (1000-1200 kcal/day) or fat restriction (22-26 g/day). Subjects participated in a 24-week behavioral weight loss program. Forty-eight subjects completed assessments at all four time points-baseline and 6, 12, and 18 months. Weight loss was significantly greater in the low calorie (LC) group (-11.2 versus -6.1 kg, p<.001) and was maintained better at the 18-month assessment (-7.5 versus -1.8 kg, p< or =.001). There was a significantly greater decrease in percent body fat and RMR for the LC group with only percent body fat still significantly lower at 18 months. Subjects in the LC group consumed significantly fewer calories, less carbohydrates, and the same amount of fat as those in the low fat (LF) condition, however, calorie and carbohydrate intake were decreasing for LF subjects by the 12- and 18-month assessments. There were no long-term differences in most measured predictors of dietary adherence. Dietary fat restriction did not prove to be superior to calorie restriction, thus strengthening the public health message that calories do count.