Two patients with Herpesvirus hominis (herpes simplex) encephalitis were investigated by serial isotope encephalograms using technetium-99m pertechnetate. In the first case the diagnosis was made by brain biopsy, and by successful tissue culture, and was confirmed by a significant rise in antibody titre, but in the second the laboratory evidence was exclusively serological. Necrotizing encephalitis was presumed in both cases because the illness was grave and focal signs developed in conjunction with radiological and electroencephalographic evidence of circumscribed lesions of the hemisphere. The emergence of new lesions in the brain scan at a time of clinical remission was found to be a warning of impending relapse. Such lesions had to be distinguished from areas of increasing uptake of isotope due only to local change in vascular permeability. Decay of EEG activity together with clinical signs of focal abnormality over an area of isotope concentration indicated a necrotizing lesion. Cytosine arabinoside (cytarabine), an alternative to idoxuridine as an antiviral drug, was used in both cases. A third patient, suspected in life of having a degenerative encephalopathy, was found at necropsy to have necrotizing encephalitis. Herpes infection though probable was not confirmed. The most severe lesions were in the frontal and temporal lobes, which had shown increased uptake of technetium in the encephalogram performed six weeks before her death.