The 24-h sleep-wake pattern of plasma LH was studied in 18 patients with severe weight loss. Sixteen patients had anorexia nervosa, one suffered from schizophrenia and one had a gastric ulcer. The age of the patients ranged from 13 to 31 years. Seventeen women and one boy participated in the study. Their weight ranged from 51 to 73 % of the ideal body weight (IBW) at the first 24-h study which was at one week after admission to the hospital. Blood was taken through an indwelling venous catheter every 30 min. The patients' weight increased under behavioural therapy and they were re-studied when they had gained approximately 10 % IBW and again prior to release from the psychiatric ward. The LH patterns were classified as infantile, pubertal or adult. All patients except for two had an iniantile pattern at the first study. Except for the two patients who had no anorexia nervosa, all developed a pubertal and/or an adult pattern. All patients whose weight was below 69 % IBW had an infantile LH pattern. Adult patterns were only seen when the body weight was greater than 80 % IBW. The increase of the average 24-h LH values was slower the older the patients were and the longer they had been anorectic.