Normal pressure hydrocephalus (NPH) is defined as a combination of dementia, gait disturbances and/or urinary incontinence, hydrocephalus, and a normal intracranial mean pressure. The clinical effect of CSF shunting in patients with this syndrome is sometimes striking, but generally only 50-60% of the shunted patients benefit from the treatment. It is assumed that the condition is caused by reduced conductance to outflow of CSF ( Cout ). A clinically usable method for the measurement of Cout has been developed. Cout has been measured in 80 patients with NPH. The results of clinical examination, computed tomography (CT), long-term intracranial pressure recording, isotope cisternography (ICG), and Cout have been compared to the clinical results of shunting 3 and 12 months after operation. Among the preoperative investigations Cout proved to have the best diagnostic specificity and sensitivity. Thus, selection of patients for shunting on the basis of Cout should lead to a satisfyingly high success rate. The different methods for measurement of Cout are discussed, and a theory on the pathophysiology of NPH is proposed. A clinical investigational programme, based on the results from clinical examination, CT, pressure recording, and measurements of Cout is suggested.