Clinical data are presented on a 30-year-old male with normal early development (4-5 years) but subsequent progressive impairment of psychomotor functions. He has marked kyphoscoliosis and talipes calcaneovalgus. The organs appear normal and the patient can walk unaided and feed himself although he does not recognize his parents. He has normal fundi oculi. Biochemical data show an absence of mucopolysacchariduria and very low but detectable levels of N-acetyl-β-D-hexosaminidase in serum and leucocytes. The clinical symptoms are much milder than would normally be expected from such a profound enzyme deficiency (Sandhoff disease).