Arachnoid cysts are considered a rare neurological tumor, few of which exhibit any symptomatology. A 38-year-old Haitian American female with no past psychiatric history presented with rapid onset of psychosis. Workup for medical etiology proved to be within normal limits, with the exception of a left temporal lobe arachnoid cyst. The purpose of this paper is to add to a number of existing case reports that suggest a relationship between such lesions and psychiatric illness.