BACKGROUND Brain arachnoid cysts are fluid collections of developmental origin. They are commonly detected incidentally in patients imaged for unrelated symptoms. METHODS A 15-year-old healthy boy with a recent history of head trauma experienced headache that gradually worsened over the course of 10 days. He underwent CT and MRI brain scans which revealed the presence of subdural haematoma caused by the rupture of a middle cranial fossa arachnoid cyst. This was accompanied by intracystic haemorrhage. The subdural haematoma was removed, while communication of the cyst with the basal cisterns was also performed. The postoperative course of the patient was uneventful. CONCLUSIONS The annual haemorrhage risk for the patients with middle cranial fossa cysts remains very low. However, when haemorrhage occurs, in most occasions it can be effectively managed only with haematoma evacuation.