A 23 year old white male, successful until 18 months prior to presentation, began noticing a decrease in mental functions which began to speed up with time. This progressed to a point where he had changes in his personality which appeared schizophreniform in nature. The patient was admitted to a general hospital for evaluation and a large arachnoid cyst was found in his left temporal lobe. He was taken to surgery where the lesion was removed. Postoperatively the patient's personality and mental status reverted back to the premorbid state.