Pure cortical arterial injury is defined as an acute subdural hematoma (ASDH) due to micro laceration of the cortical artery without cerebral contusion. We analyzed this peculiar ASDH in 14 cases. This ASDH has the following characteristics. 1) This disease develops in elderly persons after minor head trauma. 2) After a relatively long lucid interval, the consciousness level of these patients decreases rapidly to semicoma or deep coma. 3) Brain atrophy, atherosclerosis and a tendency to bleed (drugs, hemodialysis, thrombocytopenia, etc.) are involved in this hematoma. 4) The clinical outcome is generally poor because of systemic complications. The authors infer the mechanism of pure cortical arterial injury to be as follows. Minor hemorrhage occurs due to direct impact damage after trivial head trauma, but physiological hemostatic actions are set in motion. After a lucid interval, rebleeding from the injured cortical artery causes massive subdural hemorrhage.