We studied neuromuscular transmission in 16 patients with prior poliomyelitis by measuring single fiber electromyographic (SFEMG) jitter. This was compared with 3 indirect methods of assessing reinnervation: SFEMG fiber density, macro EMG, and the presence of fiber type grouping on muscle biopsy. In patients with acute poliomyelitis before the age of 10, there was a positive correlation between the extent of neuromuscular transmission impairment, demonstrated by increased SFEMG jitter, and the enlargement of the motor unit, as indicated by increased fiber density, increased macro EMG signals, and fiber type grouping on muscle biopsy. However, there was no correlation between any of these parameters and the presence or absence of new symptoms of weakness. These findings suggest that impaired neuromuscular transmission is most common in patients with prior poliomyelitis whose motor units have been maximally enlarged by axonal sprouting, but is independent of the presence or absence of new symptoms of weakness.