Conductance to outflow of CSF was measured in a prospective study of 80 patients with normal pressure hydrocephalus. Measured outflow was compared with clinical findings, computed tomography, intracranial pressure measurements and with the results of shunting. Follow-up time was one year. Outflow measurements predicted the outcome of shunting in 96 per cent of the patients. Patients with the combination of known aetiology, short history, gait disturbances, severe dementia, urinary incontinence and hydrocephalus on CT can be shunted without further investigations. Periventricular hypodensity on CT scanning predicts an excellent result for shunting. Based on results of preoperative investigations and outcome of shunting we propose a programme for the investigation of future patients with normal pressure hydrocephalus.