A 40-year-old woman presented with chronic headaches. She had undergone ventriculo-peritoneal (VP) shunt 7 years previously for treatment of hydrocephalus secondary to aqueduct stenosis. Intracranial pressure (ICP) monitoring revealed a resting ICP of less than 5 mmHg. Headaches were thought to be due to low ICP and the shunt tubing was ligated. Over the next 4 h there was an increase in ICP to 14 mmHg, decrease in GCS to 13 and ventriculomegaly on CT. These changes were reversed by the removal of the ligature. This unusual case highlights the fact that, in some shunted patients with over drainage of ventricles and a low resting ICP, small increases in ICP are poorly tolerated. This may be due to altered visco-elastic properties of the ventricles and the brain parenchyma.