Monosymptomatic delusion of infestation (parasitosis) occur in idiopathic psychoses but have rarely been observed in patients with neurologic or systemic illnesses. A patient was observed in whom the delusion occurred in the presence of demonstrable cerebrovascular disease. The delusional symptoms were contralateral to the principal site of ischemic injury. Delusions of infestation may reflect an acquired neurologic or metabolic disorder, and medical and neurologic evaluation should be pursued in elderly patients presenting with this syndrome.