Eleven patients with progressive weakness following polio (postpoliomyelitis muscular atrophy syndrome) were compared electromyographically with nine patients who had stable strength following polio. Abnormalities included (1) abnormal motor unit characteristics in many muscles indicating widespread loss of motor neurons and reinnervation in muscles, including many not clinically affected by the polio; (2) prevalent spontaneous denervation potentials; and (3) abnormal single-fiber electromyographic jitter. These electromyographic abnormalities were similar in progressive and stable postpoliomyelitis patients even when muscles were separated by strength, stability, age and duration of the postpoliomyelitis state. Postpoliomyelitis muscular atrophy appears to be the clinically apparent end of the spectrum of abnormalities existing in all postpoliomyelitis patients.