We studied three patients from two unrelated families with adult hexosaminidase A deficiency. A 30-year-old, non-Jewish proband in the first family had juvenile amyotrophic lateral sclerosis that evolved to mild dementia, ataxia, and axonal (neuronal) motor-sensory peripheral neuropathy. A 36-year-old Jewish proband in the second family had "pure" spinal muscular atrophy. One supposedly healthy brother of the first proband was found to have borderline IQ, mild spasticity, and ataxia but no evidence of motor neuron disease. Marked cerebellar atrophy was detected by head scans in all three patients. In both probands electromyograms were characterized by prominent, complex repetitive discharges in many muscles. Hexosaminidase A activities against the artificial substrate were similar to those reported in infantile Tay-Sachs disease; however, the hexosaminidase A level against GM2 substrates was higher than that found in infantile Tay-Sachs disease. The hexosaminidase A levels of the parents were in the heterozygous range. Motor neuron disease in our patients and in those previously described appears to be part of a multisystem degeneration of the nervous system.