Behaviour therapy, pharmacotherapy, and a combination of the two were compared in 120 obese women and 14 obese men during six months of treatment for obesity and at one-year follow-up. Patients who received the appetite suppressant fenfluramine lost 14.5 kg and those who received the combined treatment lost 15.3 kg, both significantly more than those who received only behaviour therapy (10.9 kg). One-year follow-up of all living patients who completed treatment showed a striking reversal of these effects: behaviour-therapy patients regained only 1.9 kg, significantly less than pharmacotherapy patients (8.2 kg) and combined-therapy patients (10.7 kg). Weight changes of the 14 men did not differ from those of the women. Although pharmacotherapy produced more rapid initial weight loss than behaviour therapy, it was followed by more rapid weight gain after treatment. Addition of pharmacotherapy apparently compromised the long-term effects of behaviour therapy. Better maintenance of weight loss and lower costs favour behaviour therapy over pharmacotherapy for the treatment of obesity.