A rare case of primary anorexia nervosa in one of prepubertal identical male twins is reported. Comparison of the twins revealed very little information about why the patient twin alone developed anorexia nervosa. A greater degree of apprehension in the father about the patient's childhood sexual curiosity may have been a factor. The patient was more driven in pursuing perfection and achievement in athletics and consequently had a greater fear of failure than his brother. However, the unaffected twin was more phobic, experienced more nightmares, and paradoxically was perceived as the less competent and more dependent twin. Rapid improvement occurred in the course of a traditional management approach and individual psychotherapy. Neither behavior modification techniques nor any sustained or systematic contact with his unyielding family was employed. The possibility is suggested that true anorexia nervosa in males more commonly occurs in preadolescence, whereas in females the onset is more common in early and middle adolescence.