Arachnoid cysts are constitutional anomalies with an asymptomatic course. There is no consensus on their etiology. They may lead to increased intracranial pressure accompanied by acute or subacute hemorrhage fenestrating into subdural space in young adults. A case of arachnoid cyst that experienced loss of consciousness one week after a head trauma and was complicated by subdural hematoma is presented. Chronic subdural hematomas are rare in young adults. An arachnoid cyst should be considered in these cases and the operation planned accordingly.