Lumbar spinal fluid drainage is a common procedure to reduce the risks of cerebrospinal fluid (CSF) fistula after skull base fractures or various transdural neurosurgical procedures. Nevertheless, this simple and effective technique can lead to overdrainage and CSF hypovolaemia. This report describes the case of a young patient who had a lumbar drain inserted, to avoid CSF fistula after a pterional craniotomy with opening of the frontal sinus for the clipping of a ruptured aneurysm. The drain was removed after 48 hours because of underdrainage (<1 ml/h). Three days after drain removal, she developed rapid deterioration of her level of consciousness and signs of cranial nerves involvement, brain stem and cerebellar dysfunction. Intracranial pressure was low (<5 cm H(2)O) and MRI showed brain sagging and cerebellar foramen magnum herniation. The patient was successfully treated with epidural blood patch, ventricular drainage, and Trendelenburg position. The authors report this case because CSF hypovolaemia attributable to lumbar overdrainage is an insidious and threatening condition not easy to diagnose in the absence of detectable CSF leak. MRI and intracranial pressure monitoring confirm the diagnosis and permit better understanding of the physiopathology of brain sagging.