Several methods are used in clinical practice to investigate herpes simplex encephalitis (HSE), including electroencephalography (EEG) and polymerase chain reaction (PCR) of the viral genome in cerebrospinal fluid. PCR is the most sensitive and specific of the diagnostic methods currently employed. We retrospectively examined the diagnostic utility of EEG and cranial imaging within the first 24-48 h of symptom onset in patients with suspected HSE. Patients with herpes simplex-positive PCR results were compared with those with herpes simplex-negative PCR results. Periodic lateralized epileptiform discharges and/or focal temporal slowing were present in 90% of the PCR-positive patients at symptom onset compared with only 30% of the PCR-negative group. The sensitivity of EEG recordings decreased after 48 h. Although no patients had computed tomography findings suggestive of HSE, magnetic resonance imaging results were consistent with HSE in 86% of those with herpes simplex-positive PCR results obtained after 48 h from symptom onset.