It is well known that arachnoid cysts can be associated with chronic subdural hematoma. We report a case of an arachnoid cyst in which the hematoma was located in the cyst wall. A 23-year-old male was found by chance to have an arachnoid cyst in the left middle fossa. We followed him up, because he had no symptoms. Two years after the initial examination, he began to complain of severe headache. MR image showed a hematoma located in the anterolateral space of the cyst. On operation, we found that arachnoid membranes had covered the hematoma. We removed the hematoma and tore the inner wall of the cyst so that cerebrospinal fluid in the cyst could flow easily into the basal cistern. Pathological examination revealed arachnoid-like tissues outside the hematoma capsules. Since the hematoma was wrapped all over by arachnoid tissues, it was considered that the hematoma was located in the outer wall of the cyst. About the mechanism of hematoma formation in this case, we suppose that some force could have split the outer wall of the cyst, and bloody fluid seeping through the wall membranes may have collected in the wall itself.