This is a 20-year-old male diagnosed in September 2003 with pre-B acute lymphoblastic leukaemia, who received intravenous induction chemotherapy and prophylactic intrathecal chemotherapy using PL. After applying various treatment regimens for 8 months, complete remission of the disease was not achieved. Four lumbar punctures for intrathecal chemoprophylaxis were performed during this period without complications. Repeated cerebrospinal fluid (CSF) analyses were normal on all occasions. Platelet counts prior to these punctures were 34,000, 118,000, 338,000 and 161,000 platelets/mm3 respectively. Prothrombin time (PT) and activated partial thromboplastin time (aPTT) were normal, and no platelet transfusion was performed prior to LP. In May 2004, the patient was readmitted to hospital for a new cycle of chemotherapy and subsequently scheduled for a bone marrow transplant. A new LP was performed without incident, but on this occasion several platelet concentrates had to be trans-fused beforehand as the patient had thrombocytopenia of 26,000/mm3. In the previous 2 months he had also required several transfusions due to a progressive tendency to thrombocytopenia (below 20,000/mm3). PT and APTT were normal. A few hours after the last LP, the patient required attention for isolated right sciatica. After about 48 hours, radicular pain was associated with loss of strength in both MMII, which rapidly evolved into bilateral paraparesis, with only partial motor strength (3/5) in flexion-extension of the feet. No sensory or sphincter disturbances were observed. An emergency dorsolumbar MRI was performed, which showed an image suggestive of ventral intradural extraparenchymal haematoma compressing conus medullaris and cauda equina from D12 to L4. After intravenous gadolinium administration, no contrast uptake suggestive of an underlying lesion was observed. Subsequently, a decompressive laminectomy was performed from L1 to L4 and a longitudinal durotomy, after which the cauda equina was observed under high tension, contained by the intact arachnoid membrane. A small amount of subarachnoid blood clot compressed the conus medullaris dorsally. Exploration of the lateral areas showed the presence of a large ventral haematoma also contained by the arachnoid membrane. The arachnoid was then incised longitudinally and after separation of the dorsal roots of the cauda equina the clot was accessed. It was only possible to achieve a subtotal removal of the clot, as it was partially organised and adhered to the nerve roots. No macroscopic lesions were observed that could be the origin of the haematoma. The surgical procedure was completed with the closure of the dural plane using a lyophilised plastia to widen the space. The postoperative course was uneventful. The patient achieved a progressive recovery of strength and was able to ambulate on the fourth day, without sphincteric alterations or radicular pain, and reported only a slight residual cramping in the lower limbs. In the following weeks, oncological treatment and bone marrow transplant were continued, but the patient developed a series of systemic complications that finally led to multi-organ failure and his death one month after the surgery. A post-surgical lumbar MRI could not be performed due to the patient's poor clinical condition.