We report 3 cases of subdural hematoma following open heart surgery under cardiopulmonary bypass. In 2 patients, emergency removal and drainage of a subdural hematoma was performed by neurosurgeons, and conservative management was performed in the remaining one. All patients belonged to a high risk group of bleeding due to anticoagulation therapy, and they had no episode of head trauma. Subdural hematomas may have been due to rapid alterations in cerebral volume, leading to a tearing of the dural bridging veins under cardiopulmonary bypass. Although early diagnosis and prompt treatment is very important, it is often difficult to examine neurological findings after cardiac surgery. We consider that patients who need long-term sedation under anticoagulation therapy must have their neurological status checked at least once in a few days.