There are many reports about complications of transphenoidal surgery for pituitary adenomas. We report a patient who suffered severe subarachnoid hemorrhage due to an indirect injury of the intradural internal carotid artery during or immediately after transsphenoidal surgery for pituitary adenoma. An autopsy suggested that a small branch of the intradural internal carotid artery was strongly adherent to the suprasellar portion of the tumor capsule and its avulsion from the internal carotid artery might have been caused as the capsule fell down during the intracapsular removal of the tumor. In a case of pituitary adenoma operated by the transsphenoidal approach the possibility of subarachnoid hemorrhage due to this type of arterial injury, though rare, should be kept in mind.