Three patients from two families had an unusual phenotypical variant of late-onset hexosaminidase-A deficiency. The clinical picture was dominated by spinal motor neuron involvement mimicking juvenile-onset spinal muscular atrophy. Atypical features included prominent muscle cramps, postural and action tremor, recurrent psychosis, incoordination, corticospinal and corticobulbar involvement, and dysarthria. The presence of these atypical features in patients whose lower motor neuron involvement would otherwise be consistent with juvenile-onset spinal muscular atrophy should raise the suspicion of the presence of hexosaminidase-A deficiency and GM2 gangliosidosis that can be proved by appropriate enzyme assays.