An elderly man presented with signs of normotensive hydrocephalus. Elevated protein content in the spinal fluid led to the diagnosis of an "asymptomatic" cauda equine neurilemoma and its removal. Within 6 weeks the patient's mental status had dramatically improved. Chronic transudation of plasma proteins including fibrinogen into the subarachnoid space had probably impeded spinal fluid reabsorption. It is suggested that the leakage of fibrinogen into the cerebrospinal fluid may be the cause of hydrocephalus in other clinical settings in which there is an elevation of the spinal fluid protein.