THE clinical features of an extradural hTmatoma due to rupture of the middle meningeal artery are well known; the " classical" picture may, however, be frequently absent or overshadowed by the signs produced by single or multiple cerebral lesions by a second extradural or a subdural haematoma. Besides the meningeal arteries the great venous sinuses can produce intracranial heemorrhages. If a tear involves such a sinus and gives rise to an extradural hematoma overlying one of the more or less " silent" cortical areas, early diagnosis may be very difficult. To this latter group belongs the extradural haematoma of the posterior fossa; probably the rarest form of traumatic intracranial bleeding. The present paper contains a description of such a case, which was under the writer's care in spring, 1938. A girl, aged 24, was thrown off her bicycle by a blow from an opening door of a stationary car, which she was about to pass. She struck the back of her head heavily, fell, and lost consciousness for a few minutes. Later she vomited, but on admission to hospital shortly after the accident she only complained of severe headache. There were no external injuries and no neurological abnormality was detected. X-ray pictures showed a slight separation of the left half of the lambdoid suture. About 30 hours later an experienced ward-sister observed the patient in a series of typical " cerebellar fits." In these her head was well retracted and her extremities rigidly extended; she was cyanosed, her breathing was irregular and she rapidly became unconscious. The first attack lasted about 2 minutes, a second one occurring a few hours later, and a third prolonged and severe attack was observed on the morning of the third day. After the first attack the patient's condition, which had been satisfactory until then, changed for the worse. She became increasingly restless and finally comatose. A lumbar puncture was performed, but only a few drops of clear cerebro-spinal fluid were obtained, the flow then ceased abruptly. A second puncture, performed 2 hours later, yielded the same result. There was no nystagmus and the tendon reflexes were equal on both sides. The history of trauma, the separation of the left half of the lambdoid suture, the cerebellar fits, the cerebro-spinal fluid block, and the increasing coma all pointed to a htmorrhage in the posterior fossa, probably extradural. Owing to the patient's deterioration operation could no longer be delayed.