Virologic studies were done on 511 patients with suspected viral disease of the central nervous system. In 368 cases of aseptic meningitis, infections were encountered with a total of 20 different enteroviruses (of which Coxsackie B5 was the most frequent) and with the mumps and herpes simplex viruses. Poliovirus infection was established in 26 of 35 patients (75%) with paralytic illness who had not been immunized, and in 3 of 17 patients (18%) who had received 3 doses of Salk vaccine. Other agents associated with paralytic illness included Coxsackie virus types B2, B3, B5, ECHO 9 virus, and the mumps virus. In 33 of 60 patients with clinical encephalitis, no viral infection was detected; in the remainder, infections with an array of different entroviruses, the mumps, and herpes simplex viruses were encountered.