Pre-existence of an arachnoid cyst has previously been suggested to increase the risk of developing a subdural haematoma (SDH). In this case a young man was submitted acutely due to decreasing level of consciousness. Four weeks previously he had been hit by a football and suffered minor and transcient headache. Within minutes after submission, a further decrease in consciousness according to the Glasgow comascore was observed. Acute computed tomography showed SDH and an arachnoid cyst. Surgery was performed immediate and hours later he was improving.