The effect of surgical treatment of patients with normal pressure hydrocephalus (NPH) is reported to be variable. Candidates for surgery have often been selected using hydrodynamic tests; however, the predictive value of such tests remains uncertain. Seventeen patients with idiopathic NPH underwent continuous computerized intracranial pressure (ICP) monitoring and a steady state lumbar infusion test determining the resistance to cerebrospinal outflow (R(out)). The patients were treated surgically and clinically re-examined approximately 6 months postoperatively using a new NPH score. The effect of surgical treatment was good. The R(out) was positively correlated with the clinical state of the patients before treatment. After surgery, the R(out) correlated well with the improvement in gait and NPH score. Ventricular size was negatively correlated with hydrodynamic variables before treatment. Surgery reduced the ventricular size only slightly and the degree of reduction was linked to the R(out). ICP measurements could not predict outcome in NPH cases.