This article describes the neuropathologic findings in a woman who died suddenly at the age of 48 after having had symptoms of narcolepsy for 1 1/2 years. Gross examination of the brain showed an arachnoid cyst. The significant light microscopic findings consisted of focal gliosis of the ventrolateral caudal pons and the periventricular anterior hypothalamus. Although cases of symptomatic narcolepsy have been previously reported, to our knowledge structural brain changes in idiopathic narcolepsy have not been previously described. In the present case, there is no evidence that the narcoleptic symptoms were secondary to any other disorder, including the arachnoid cyst. This patient's symptoms may be related to the pontine and hypothalamic gliosis, the etiology for which is unknown.