Macular cherry-red spot, myoclonus and progressive mental deterioration are described in a man of 16 years. Morphological examination of the liver, bone marrow and fibroblasts showed numerous vacuoles containing storage material in the cytoplasm of the cells. Twelve different oligosaccharides were isolated from urine and their structures were determined. All have N-acetylglucosamine in a reducing end and (2--3) and 2--6) neuraminic acid in the terminal position. This abnormal urinary oligosaccharide excretion is due to absence of (2--6) neuraminidase which was not detected in fibroblast culture. This case is discussed in relationship to other cases with macular cherry-red spot, myoclonus and oligosaccharide urinary excretion.