A case of acute spinal subdural hematoma following lumbar puncture is reported. A 52-year-old male was admitted for evaluation of persistent headache after head injury. Computed tomography (CT) scan showed dilated ventricles and a ventriculo-peritoneal shunt was placed. Preoperative coagulation studies were normal and no difficulty of hemostasis was encountered during the surgery. Three weeks following the surgery, lumbar puncture was performed for CT cisternography. A few hours after the lumbar puncture, he complained of low back pain. The pain persisted the next day and radiated down to the lower extremities. Examination revealed positive straight leg raising test on the left side. The following morning he became paraplegic with urinary retention. An emergency myelography by lateral C 1/2 puncture revealed complete blockage at the level of Th10. Laminectomy disclosed a large subdural clot extending from the sacrum to the mid-thoracic region. No specific bleeding point was identified. Postoperatively he remained paraplegic.