Three cases of acute subdural hematoma related to bleeding aneurysm or arteriovenous malformation are presented. Various possible arterial sources of bleeding that may lead to subdural hematoma are reviewed. The complex cause and effect relationship between the presence of aneurysm or arteriovenous malformation and the occurrence of subdural hematoma in a head-injured patient is discussed. The pitfalls of reliance on the computed tomogram as a sole diagnostic test in the evaluation of head-injured patients or of unconscious patients with no available history are emphasized.