Two adult Ashkenazi Jewish siblings have had slowly progressive deterioration of gait and posture since early childhood, distal to proximal muscle atrophy, pes cavus, foot drop, spasticity, mild ataxia of limbs and trunk, dystonic features, and dysarthria. Vision and optic fundi are normal, verbal intelligence is stable, and no seizures have occurred. The sister of the patients died at 16 years of age with the same illness. Autopsy showed diffuse neuronal storage, predominating in subcortical areas, consisting of membranocytoplasmic bodies, zebra bodies, and complex lamellar structures. GM2 ganglioside was increased in her brain. Hexosaminidase A was decreased in serum and leukocytes of the living patients, and was in the range for carriers of Tay-Sachs disease in their parents. The disease found in this family represents a new, more indolent variant of GM2 gangliosidosis.