The authors report two cases of rapid resolution of acute subdural hematoma. Case 1, a 78-year-old male, sustained head trauma and became unconscious for a few minutes, but on arrival he was alert and neurological examination was normal. A computed tomography (CT) scan taken 5 hours after the injury showed a high-dense subdural hematoma partly containing low-density area over the right cerebral hemisphere. He was conservatively treated. A CT scan taken 33 hours after the injury disclosed diminution of the hematoma and appearance of a thin high-density layer in the subdural space adjacent to the tentorium. The high-density layer disappeared on a CT scan taken 7 days after the injury. Case 2, an 84-year-old female, struck her head in the occipital region. On arrival she was drowsy but otherwise had no neurological abnormality. A CT scan taken 1 hour after the injury demonstrated a high-dense subdural hematoma partly containing low-density area over the right cerebral hemisphere. She was conservatively managed and became asymptomatic. A CT scan taken 14 hours after the injury disclosed complete resolution of the hematoma and appearance of a thin high-density layer in the subdural space adjacent to the falx and the tentorium. This high-density layer also completely disappeared on a CT scan taken 14 days after the injury. It was suggested that a mechanism of rapid resolution of the acute subdural hematoma was attributable to washing-out by cerebrospinal fluid.