Detailed clinical and biochemical studies are reported in two siblings with isovaleric acidemia ("sweaty feet syndrome"). The younger of these siblings was begun on treatment with a low-leucine diet at the age of 17 days and was the subject of a previous brief report (Pediatrics, 49: 616, 1972). Within one day of the beginning of treatment urinary isovaleric acid dropped from 1661.4µmoles/24 hr to 94.1µmoles/24 hr and urinary isovalerylglycine dropped from 480 mg/23 hr to 150 mg/24 hr. Until the age of 3 years he was maintained on this diet and had many less episodes of vomiting and acidosis than his affected but untreated sister had had at corresponding ages. Psychosocial functioning has been at low normal levels whereas the affected sister has a moderate degree of mental retardation. It is postulated that isovaleric acidemia may be added to the growing list of disorders that can cause mental retardation but that may be amenable to dietary therapy begun in early infancy.