DURING the past two years, we have studied a brother and sister, now aged 3½ and 5 years, with a previously-undescribed disorder of leucine metabolism resulting in excessive accumulation of isovaleric acid in the body fluids1 (Fig 1). Both children have mild mental retardation, smell of isovaleric acid (the odor of sweaty feet), and experience periodic bouts of vomiting, acidosis, and lethargy or coma. These attacks are usually associated with infection or immoderate protein ingestion, and are characterized by marked intensification of the odor and an increase in the blood concentration of isovaleric acid. With glucose and electrolyte therapy, the attacks have subsided gradually over a few days; during the recovery phase, ataxia and intention tremor have been noted. The only other abnormal physical findings are mottling of the fundus and congenital retinal vessel tortuosity in both patients. Neither the parents nor the normal brother have these features.