We report a case of a patient with an arachnoid cyst in which subdural hematoma and intracystic hemorrhage developed spontaneously. Usually, arachnoid cysts are asymptomatic, but can become symptomatic because of cyst enlargement or hemorrhage, often after mild head trauma. Although they are sometimes combined with subdural hematoma, intracystic hemorrhage has rarely been observed. Our patient had a simultaneous subdural hematoma and intracystic hemorrhage without evidence of head trauma.