The difference in the clinical, electrophysiological and radiological features in 12 patients with Japanese encephalitis (JE) and eight patients with herpes simplex encephalitis (HSE) have been reported in this study. Meningeal signs, seizures and behavioural abnormalities in HSE; and decerebration or decortication and focal neurologic signs in JE were common. Electroencephalogram in JE revealed diffuse delta slowing in 11, whereas in HSE frontotemporal slowing was present in five, periodic lateralising epileptiform discharges in three and focal spikes in two patients. Magnetic resonance imaging in JE patients revealed characteristic bilateral thalamic hyperintense lesions in T2 in all the patients. In HSE, CT scan revealed frontotemporal hypodensity in six patients. MRI was more sensitive than CT scan. It revealed characteristic frontotemporal hyperintense signals in T2 even in two patients who had normal CT scan. Motor evoked potentials were abnormal in eight out of nine JE patients, whereas these were normal in all five HSE patients in whom these studies were carried out. Attention to these clinical, radiological and neurophysiological findings may help in differentiating these encephalitides even before the results of serological studies are available.