In-situ hybridisation with a biotinylated cytomegalovirus (CMV) DNA probe was done on brain biopsy specimens from 10 patients with Rasmussen's encephalitis (RE) and 46 agematched control patients with other neurological diseases. All 10 patients with RE had intractable epilepsy and focal neurological deficits, and there was perivascular cuffing, microglial nodules, astrogliosis, and neuronal loss. CMV genomic material was demonstrated in 7 of the 10 patients with RE (in neurons, astrocytes, oligodendrocytes, and endothelial cells) and in 2 of the 46 control patients. Probes for herpes simplex virus and hepatitis B virus were negative in all patients and in fibroblast controls. The results suggest that CMV is a likely cause of Rasmussen's encephalitis.