The CT findings in six patients with confirmed herpes simplex encephalitis were analysed and compared with the literature. Thirty-four examinations were performed, 27 within the first 14 days of the illness. The early findings show three characteristic features: The CT may be entirely normal up to the fourth day. Consistent with clinical and E.E.G. findings there then develops a hypodense temporal lesion which, even at this stage, acts as an expanding lesion in half the patients. Between the sixth and eighth day there is frequently involvement of the contra-lateral temporal lobe, not as a continuation, but as a new lesion. At the same time, or after several days, there is involvement of the basal portions of the frontal lobes. In this later phase there may be necrosis in the basal ganglia, thalamus or other parts of the brain. The CT findings in this late phase are uniform and characteristic of the disease. For early diagnosis and treatment the early clinical, electrophysiological and neuro-radiological findings are important. A normal CT scan two days after the onset of clinical symptoms may be regarded as significant.
