We report here a case of juvenile metachromatic leukodystrophy. The patient is an 8-year-old boy with motor and mental deterioration, which began at about age 3. He has also suffered from astatic seizures since age 8. Arylsulfatase A activity in the patient was markedly decreased in peripheral leukocytes, cultured fibroblasts and urine. Sulfatide was detected in urine from the patient by thin-layer chromatography. Peripheral motor and sensory nerve conduction velocities were markedly reduced. Computerized tomography of the brain showed low density areas in the periventricular white matter which were not enhanced by intravenous contrast material. His parents' arylsulfatase A activities were about half those of normal controls. This is the third case of juvenile metachromatic leukodystrophy in Japan.