Cerebrospinal fluid specimens from 226 patients with suspected viral infections of the central nervous system (CNS) were tested by polymerase chain reaction (PCR) to identify the most frequent viruses involved in these infections. A positive PCR result was obtained in 18 patients (7 cases positive for herpes simplex viruses, 5 for enterovirus, 3 for JC virus, 2 for varicella-zoster virus, and 1 for Epstein-Barr virus). All patients with positive PCR results had a definite diagnosis of CNS viral infection. However, a negative result did not rule out the possibility of viral infection of the CNS.