Arachnoid cysts of the middle cranial fossa may present clinically in several different ways. They may remain asymptomatic throughout life, or may cause only local bulging of the skull, with or without exophthalmos. Expansion of the cyst may occur through a ball-valve mechanism of its membrane in communication with the general subarachnoid space leading to increased intracranial pressure with or without localizing neurological deficits. Finally, such cysts are associated with increased vulnerability to the effects of head injury, resulting in hemorrhage either into the cyst or external to the cyst to form a subdural hematoma. Recognition of these various manifestations is essential to proper treatment.