A patient with Kleine-Levin syndrome, typical except that onset was at 39 years of age, died during a symptomatic period. Autopsy disclosed recent and old lesions in the medial thalamus involving intralaminar, medial, and some dorsal nuclei as well as the pulvinar. Despite massive microglial infiltration, there was minimal neuronal loss. The hypothalamus was not involved. The findings suggest a viral cause for Kleine-Levin syndrome.