A retrospective clinical and pathological analysis has been performed of 24 cases of herpes simplex virus encephalitis (HSE) seen at the Institute of Neurological Sciences, Glasgow, between 1972 and 1985. All patients had been diagnosed on the basis of isolation of herpes simplex virus (HSV) from, and/or the demonstration of characteristic histological changes of acute necrotizing encephalitis (ANE) in brain biopsy and/or autopsy tissue. Clinical presentation on admission included a prodromal influenza-like illness (46%), sudden onset of headache and confusion (54%), meningism (38%), deep coma (42%), aphasia (54%) and focal neurological signs (79%). Seizures occurred in 46% of cases during the course of the illness. Of the 24 cases, 14 (58%) died and 10 (42%) survived. Intravenous acyclovir treatment was associated with the best prognosis. Cerebral biopsy of one temporal lobe was performed in 22 cases and in 19 of these a positive histological diagnosis of HSE could be made. HSV was isolated from 15 of the 19 (79%) biopsied cases in whom virus isolation was attempted. Only seven out of the 15 cases (47%) in which immunofluorescence assays for HSV antigens were performed were unequivocally positive. Herpes simplex virus was isolated in culture from all cases which were negative by immunofluorescence. Immunocytochemical analysis on tissue sections of five representative brain biopsies demonstrated the presence of HSV antigens in some astrocytes, neurons and macrophages especially within areas of inflammatory infiltration. In situ hybridization experiments with a cloned HSV DNA probe demonstrated viral RNA in astrocytes, neurons and macrophages in two human biopsies and mouse brains in areas broadly corresponding to the distribution of viral antigen labelling. The combined immunocytochemical and in situ hybridization procedure showed that many but not all of the cells containing viral RNA also contained HSV antigens, indicating a productive infection in these double-labelled cells.