We describe four cases of postpolio syndrome with typical histories, physical examination results, and electrodiagnostic evidence of extensive anterior horn cell disease, as well as the putative pathophysiology of postpolio syndrome in persons with histories of nonparalytic polio and the diagnostic implications for individuals older than 40 yr of age who are experiencing unexplained new weakness, fatigue, and muscle or joint pain. Although the diagnosis of postpolio syndrome traditionally has required a remote history of paralytic polio, many persons such as the ones described here with typical symptoms of postpolio syndrome have no clear history of paralytic disease and are being misdiagnosed. With this in mind, we believe that the diagnostic criteria for postpolio syndrome should be modified to include the following: a history of remote paralytic polio or findings on history, physical examination results, and laboratory studies compatible with poliovirus damage of the central nervous system earlier in life.