After the introduction of 4-methylumbelliferyl-2-acetamido-2-deoxy-beta A-D-glucopyranoside (4MUG) and its sulfated form (4MUGS) in the pre- and postnatal diagnosis and carrier identification of gangliosidosis genotypes, infrequent forms of the GM2 gangliosidosis Type B (Tay-Sachs disease) have been observed which show normal activity of Hexosaminidase A (Hex A) isoenzyme with the substrate 4MUG but absent or deficient activity against the sulfated form 4MUGS. Here we report the observation of a German/Hungarian boy aged 12 when he died with a prolonged course of a neurodegenerative disorder, later biochemically identified as a GM2 gangliosidosis B1-variant which is characterized by a deficient Hex A activity only against 4MUGS. The first clinical symptoms had occurred after the age of 14 months with a clear manifestation of the disease at age 3, when he presented disturbances of movement and tended to fall down. The slowly progressive course with brain atrophy, seizures and severe mental deterioration resulted in death after almost 9 years. At autopsy, the typical light microscopic neuronal changes of a "lysosomal storage disorder" were found, with multilamellar concentric bodies (MCB) and Zebra bodies in the neuronal cytoplasm at the electron microscopic level.