Three adult siblings had atypical progressive spinal muscular atrophy of the limb-girdle type, predominantly sensory polyneuropathy and cerebellar ataxia. Hexosaminidase A and B activity was profoundly decreased in serum, leukocytes and cultured fibroblasts. GM2-gangliosidosis, variant O (Sandhoff disease) was diagnosed. Mechano-allodynia was the presenting symptom in two of the patients. After 50 years of disease evolution, the patients led an independent life and were intellectually normal. The literature on the adult form of GM2-gangliosidosis is reviewed.