A new inborn error of metabolism, characterized by urinary excretion of substantial amounts of β-hydroxyisovaleric acid and β-methylcrotonylglycine, is described. The disorder is most likely due to a defect in the biotin-dependent enzyme β-methylcrotonyl-CoA carboxylase in the leucine degradation pathway. The patient, a 4½-month-old girl, suffered from neurological symptoms similar to those of Werdnig-Hoffmann's disease. No episodes of acidosis could be detected. In periods the urine had a peculiar odor, like that of cat's urine. Relatively large doses of biotin (0.25 mg/day) did not influence the condition. On a diet containing the minimum requirement of leucine, the daily excretion of β-hydroxyisovaleric acid dropped from 400 mg to about 50 mg and β-methylcrotonylglycine from 100 mg to 50 mg. However, the clinical course was unaltered and the patient died at 9 months of age from bronchopneumonia. Biochemically, as well as clinically, the findings are distinctly different from previously described errors in the leucine metabolism.