Muscle and platelet thermogenesis was measured by direct microcalorimetry in patients with anorexia nervosa. The median heat production, as calculated per unit of tissue mass, was 0.35 mW/g in vastus lateralis muscle, which is significantly lower (p less than 0.001), by approximately 50% than that in matched healthy subjects, 0.74 mW/g. The average deficit in body weight was 35%. Platelet heat production was decreased to a lesser degree with median values being 51.3 fW per cell and 60.9 fW per cell, respectively (p less than 0.02). Patients with bulimia without concomitant anorexia nervosa had normal heat production values. In the control group a positive correlation appeared (rs = 0.75, p less than 0.02) between resting muscle heat production and body surface area whereas no such relationship was found in the anorexic group.