Five patients with hydrocephalus who failed to respond to apparently adequate CSF drainage via a functioning shunt (four cases) or external ventricular drain (one case) are described. In three of the four shunted cases, the shunt was ventriculoperitoneal with a medium pressure valve, and in one a combination of peritoneal and atrial shunts both with low pressure valves. All five patients were tested for possible low pressure hydrocephalus by a period of external ventricular drainage at heights of 0 to -5 cm H2O below the reference point (external auditory meatus--EAM). Four of the five patients showed rapid and significant clinical improvement and went on to shunt revision (three) or insertion (one). The shunts were then all peritoneal, of which three were valveless, whilst one had a Sophy programmable valve at the lowest setting. In all four patients the improvement was sustained and was associated with a radiological (CT or MRI) improvement which varied from marked to slight. In the fifth patient there was no improvement with low pressure external drainage and no shunt revision was undertaken. On the basis of these cases the possible entity of low pressure hydrocephalus is discussed with particular reference to mechanism, recognition and management.