A 56-year-old man treated with anticoagulants complained of a gradually worsening headache. A left chronic subdural hematoma (CSH) was shown by head computed tomographic (CT) scans and the operation, one burr hole surgery under local anesthesia, itself was performed uneventfully. However, immediately after we began draining the hematoma at the patient's bedside, the patient complained of a sudden headache. CT scans showed a subarachnoid hemorrhage (SAH). Cerebral angiography was immediately performed, but the source of hemorrhage could not be found. The next day, a CT scan showed that most of the SAH had disappeared. To our knowledge, there are no previous reports of SAH of unknown origin following surgery for CSH. The likely mechanism for the occurrence of the SAH, in addition to a coagulopathy due to anticoagulant therapy, could include the possibility that the drainage of the hematoma produced a movement of the hemisphere along with hyperperfusion that resulted in the rupture of a weak subarachnoid vessel, such as a perforating artery.