Restricting dietary fat intake while consuming carbo hydrates ad libitum has recently been promoted as a weight loss regimen. Sixty subjects (52 females and eight males) were randomized to low fat ad libitum carbohydrate (low-fat) or low fat with caloric restriction (low-calorie) behaviour modification treatments. Forty-nine subjects completed the 16-20 week programme. Subjects in both groups reported averaging over five exercise sessions per week during treatment. The low-calorie group lost significantly more weight (males 11.8 kg, s.d. 6.4; females 8.2 kg, s.d. 4.2) than the low-fat group (males 8.0 kg, s.d. 1.3; females 3.9 kg, s.d. 3.7). Both groups of subjects lost similar amounts of lean body mass. There was significantly greater loss of body fat in the low-calorie group. A slight reduction in RMR, adjusted for changes in lean body mass, was observed in both groups. Fat intake was reduced from 90 to 30 g per day. Subjects in both groups reduced their total energy intake with the low-calorie group consuming fewer calories per day than the low-fat group. Both groups showed significant and equivalent improvements in eating habits based on microanalysis of eating diaries. Eating in social situations and emotional eating, however, continued to cause adherence problems during treatment for both groups. Follow-up data collected 9-12 months after completion of treatment on 65% of the subjects completing the study showed no significant difference between the two groups in weight losses from baseline (low-fat group 2.6 kg; low-calorie group 5.5 kg).