"Post-polio syndrome" defines the new muscle weaknesses and the variety of new difficulties with daily living that some patients experience 25-35 years after maximum recovery from acute paralytic poliomyelitis. The new weakness appears to be due to disintegration of the distal nerve terminals of the surviving motor neurons whose soma has been stressed for years to maintain large motor units via excessive distal sprouting. The factors responsible for the disintegration of axonal sprouts and manifestation of the new weakness and fatigue are unknown. Whether an immune response to the poliovirus, immunogenetic, dysimmune or aging factors play a role in the pathogenesis of this process is also unknown.