Between 1983 and 1991, 16 cases of herpes simplex encephalitis were diagnosed at the Royal Children's Hospital, Melbourne by virus isolation from the brain or cerebrospinal fluid (CSF) (2 cases), by detection of herpes simplex virus-specific IgM, IgA or IgG by enzyme immunoassay (12 cases) or by polymerase chain reaction (PCR) and herpes simplex virus-specific antibodies (2 cases). Specific antibody was detected in 4 of 13 CSF samples taken on Days 1 to 4 after onset of neurologic symptoms compared with 15 of 17 samples taken after the fourth day of illness. PCR was retrospectively applied to 20 stored CSF samples from 11 patients; 5 of 8 samples taken less than 4 days after onset of symptoms were positive compared with 2 of 12 taken after Day 4. In contrast all 5 fresh unfrozen CSF samples taken from Days 2 to 21 were positive by PCR. These results indicate that PCR is more sensitive for early diagnosis of herpes simplex encephalitis than detection of specific antibody