Viral studies were made of 1,407 patients with clinical manifestations of infectious disease involving the central nervous system. Both virus isolation and serologic methods were used. The results showed apparent differences in the etiology of clinical poliomyelitis and aseptic meningitis which correlated with the degree of prior immunization with inactivated poliovirus vaccine. Poliovirus was identified less often, and other viruses more often, among patients with both paralytic and nonparalytic illness who had received the vaccine. The findings also indicated that "nonpolio" enteroviruses have protein pathogenic capabilities enabling them to give rise to a broad spectrum of syndromes embracing the clinical picture of aseptic meningitis, mild to moderate paralytic poliomyelitis, and meningoencephalitis.