In a 4-year prospective follow-up study of 25 anorectic and 24 bulimic adolescents (DSM-III-R criteria), we studied fluctuations and differences in the course of illness, using repeated measurement. Morgan & Russell general outcome categories on the Outcome Schedule were more favourable than in adult studies. Of all patients, 47% had good, 43% intermediate and 10% poor outcome scores after 4 years. The groups differed significantly as to the kind and severity of disturbed eating behaviour and its consequences, but were rather similar in mental, psychosocial and psychosexual symptoms during the course of the illness. Eight percent of the anorectic patients became bulimic. We conclude that anorexia nervosa and bulimia nervosa can be considered as different symptom patterns of one basic eating disorder, in which preoccupation with food and a disturbed body image are core symptoms.