Unlike cytomegalovirus (CMV) ventriculoencephalitis, herpes simplex virus type 1 necrotizing encephalitis has only rarely been observed in AIDS patients. A 40-year-old bisexual man was followed for an HIV1 infection from 1987 onwards. In June 1993 he was referred for sudden confusion, left hemiparesia and fever. The blood contained less than 10 CD4 lymphocytes/mm3. The patient remained comatose and febrile, and died 4 weeks later. In coronal sections of the brain there was necrosis of the internal parts of the left temporal lobe, necrosis of certain areas of the ventricular walls and a small tumor at the top of the right frontal lobe, which proved to be a polymorphic high-grade lymphoma. CMV ventriculoencephalitis lesions were prominent in the ventricular walls of the occipital lobes and there was a strong nuclear signal for CMV using in situ hybridization. Herpes simplex virus type 1 was shown in the nuclei and cytoplasm of certain neurons and astrocytes in the borders of the necrotized temporal lobe areas by immunohistochemistry, in situ hybridization and electron microscopy, whereas in situ hybridization and immunohistochemistry for CMV were negative in such areas. Necrotizing type 1 encephalitis must not be overlooked in immunodeficient patients.