Studies were made on whether body weight loss in patients with muscular dystrophy is due to reduced intake and/or abnormal expenditure of energy. For this, food intakes and various physiological variables were surveyed in totals of 310 patients with Duchenne muscular dystrophy (DMD) of 11 to 29 years old and 28 patients with limb-girdle muscular dystrophy (LGMD) of 30 to 47 years old. Energy and protein intakes, expressed on a unit body weight basis, in DMD patients were comparable to, or higher than the allowances for age-matched healthy controls, whereas those in LGMD patients were 92 and 94% respectively of these allowances. The basal metabolic rate (BMR), expressed as kcal/kg/day, of DMD patients of all ages was higher than that of controls, the difference increasing with age, and being about 20 to 30% higher than that of controls in older patients with DMD. The BMR of LGMD patients was nearly normal. The maintenance requirements of conven-tional dietary protein in DMD and LGMD patients were 1.26 and 0.84g/kg/day, respectively. These values were about 68 and 12% higher than the normal adult value (0.75g/kg/day), indicating decreased protein utiliza-tion and increased protein catabolism. Daily excretion of urinary 3-methylhistidine (3MH) per unit muscle mass (μg/mg creatinine) by MD patients was significantly higher than that by controls, indicating in-creased degradation of muscle protein. The BMR, maintenance protein requirement and 3MH excretion of DMD patients suggest that DMD is a hypercatabolic disease. Comparison of the energy and protein intakes with the allowances estimated in consideration of increased requirements showed deficiencies of energy and protein in DMD patients. Thus, we conclude that the underweight of the DMD patients resulted from nu-trient deficiencies due to hypercatabolism, despite their considerably high intakes of energy and protein, expressed as per kg body weight. These deficiencies were confirmed by demonstrating decreased concentrations offree essential amino acids, particularly branched chain amino acids, in their serum. The values of variables of LGMD patients were intermediate between those of DMD patients and control subjects.