The herpes simplex virus produces a severe localized necrotizing encephalitis of specific areas of the brain, usually the temporal or adjacent portions of the frontal lobes. This often results in an abnormal brain scan and carotid angiograms, showing a mass not observed in other central nervous system viral infections. Positive diagnosis is made by brain biopsy and recovery of the virus. The efficacy of the antiviral agent idoxuridine is as yet unproved, but its introduction has heightened interest in earlier, more accurate clinical diagnosis. This report concerns 4 cases which showed temporal or frontal mass effects at angiography. The 2 cases in which the virus was obtained during life also showed abnormal radionuclide images. Demonstration of the evolution of these abnormalities on serial scans and angiograms confirms the rapidity of the necrotizing process in herpes simplex encephalitis.