Bladder function in four patients with normal pressure hydrocephalus, one with dementia of Alzheimer type and five patients with multi-infarct dementia was studied by history and urodynamic tests (cystometry and Bor's ice water test). The bladder hyperactivity could be temporarily improved by a lumbar puncture and removal of 50 ml CSF and later abolished by a shunt operation in patients with normal pressure hydrocephalus while no changes occurred in the other patients. Urodynamic testing in connection with a lumbar puncture may be a reliable way to predict the outcome of a shunt operation in cases of normal pressure hydrocephalus.