Thirty patients were selected for a prospective study according to two criteria: (i) an irresistible urge to overeat (bulimia nervosa), followed by self-induced vomiting or purging; (ii) a morbid fear of becoming fat. The majority of the patients had a previous history of true or cryptic anorexia nervosa. Self-induced vomiting and purging are secondary devices used by the patients to counteract the effects of overeating and prevent a gain in weight. These devices are dangerous for they are habit-forming and lead to potassium loss and other physical complications. In common with true anorexia nervosa, the patients were determined to keep their weight below a self-imposed threshold. Its level was set below the patient's healthy weight, defined as the weight reached before the onset of the eating disorder. In contrast with true anorexia nervosa, the patients tended to be heavier, more active sexually, and more likely to menstruate regularly and remain fertile. Depressive symptoms were often severe and distressing and led to a high risk of suicide. A theoretical model is described to emphasize the interdependence of the various symptoms and the role of self-perpetuating mechanisms in the maintenance of the disorder. The main aims of treatment are (i) to interrupt the vicious circle of overeating and self-induced vomiting (or purging), (ii) to persuade the patients to accept a higher weight. Prognosis appears less favourable than in uncomplicated anorexia nervosa.