Aseptic meningitis, the most common infection of the central nervous system, is an acute illness mostly caused by enteroviruses. Cerebrospinal fluid(CSF) has been used for the detection of enteroviral RNA but the detection has been mostly performed in a single CSF specimen obtained during the illness. A major objective was to evaluate the relation of sampling time to the recovery of enteroviral RNA in CSF. Thirty seven CSF specimens were obtained from 24 patients between May and August 1993, when an outbreak of asceptic meningitis by echovirus type 9 occurred. Enteroviral RNA in CSF was detected by polymerase chain reaction(PCR). Data about onset of symptom development were obtained by review of medical records. Enteroviral RNA was detected by PCR in 29 of 37 CSF specimens. PCR yielded positive results in 4 of 5 CSF specimens obtained on day 1 to 3, 10 of 11 on day 4 to 6, 8 of 10 on day 7 to 9, 6 of 8 on day 10 to 12, 1 of 3 on day 13 to 15 postonset. Of 11 patients from each of whom more than one CSF were obtained on different day postonset, PCR yielded positive resutls in 2 of 3 cases in whom enteroviral RNA detection was negative in the first CSF. These results indicate that two or more CSF specimens obtained within 12 days postonset are required for improving the accuracy of the diagnosis of enteroviral meningitis.