In some adult patients with dementia and/or disorders of gait, symptoms have been related to communicating hydrocephalus: isotope encephalography demonstrates an associated CSF circulation/absorption abnormality. A simple manometric test involving pressure readings after removal and re-introduction of CSF is described. It has shown a distinctive pattern, correlating with radio-isotope studies in 14 patients, in the separation of this condition from other causes of adult dementia. The test also gave useful information in 2 of these patients concerning patency of the ventriculoatrial shunt apparatus.