A Japanese male with juvenile Sandhoff disease is described. The patient was a product of full-term normal pregnancy from non-consanguineous parents. Since age 10, he developed progressive dysarthria and proximal muscle atrophy and weakness. Mental deterioration and cerebellar ataxia are also noted since the age of 20. On neurological examination at age 35, he showed decreased mentality (IQ 62), marked atrophy and weakness of proximal muscles, cerebellar ataxia and increased deep tendon reflexes. Brain CT scans revealed moderate to marked atrophy of cerebellum. Giant MUP, fasciculation potentials and positive sharp waves were observed on EMG examination. Biopsied sural nerve showed markedly decreased myelinated fibers. Hexosaminidase A and B activities in leukocytes and cultured fibroblasts were about 10% of normal values, while other lysosomal enzyme activities were within normal range. Rectal biopsy demonstrated lamellar inclusion bodies in submucosal ganglion cells. This is the first Japanese patient with juvenile Sandhoff disease presenting symptoms similar to motor neuron disease and cerebellar degeneration.