Macro-electromyography (macro-EMG) studies have provided important information about the size of the motor units and the degree of reinnvervation in clinically affected muscles of patients with a history of poliomyelitis and postpolio syndrome. The study of clinically unaffected muscles and correlation of their electrophysiologic characteristics with the muscle architecture could provide meaningful information about the ongoing subclinical denervation. We performed macro-EMG and concomitantly measured fiber density in the clinically unaffected gastrocnemius muscle of 10 patients with postpolio syndrome and 10 normal subjects of similar age. We also performed biopsies on the gastrocnemius muscle of 8 of the patients. The median amplitude and area of the macro-motor unit potentials (macro-MUPs) were increased in 8 of the 10 patients, and occasionally were five times as large as the mean median value for the normal subjects. Seven biopsy specimens showed moderate to very large fiber-type grouping. In 5 patients, there was correlation between the electrophysiologic and histologic indices of reinnervation. Amplitude and area of the macro-MUPs were associated with the muscle fiber cross-sectional area. We conclude that clinically unaffected muscles of patients with postpolio syndrome often have large motor units as the result of effective reinnervation after the original motor neuron loss. In spite of possible differences in the cytoarchitecture of muscles affected to different degrees, macro-EMG and fiber density measurements are reliable noninvasive techniques for studying the extent and effectiveness of reinnervation in patients with postpolio syndrome. 0 1996