It is well known that in patients with communicating hydrocephalus or normal pressure hydrocephalus, ventricular volume decreases following implantation of differential pressure valved shunts. Hydrostatic valves (Miethke dual-switch valves) were implanted in 60 normal pressure hydrocephalus patients at Unfallkrankenhaus Berlin between September 1997 and September 2001. One year postoperatively, these patients underwent CT scan, and their ventricular size was ascertained using the Evans index. Although 77% of these patients showed no postoperative change in ventricular volume, 65% nonetheless showed good to excellent clinical improvement, 13% satisfactory improvement and 22% no improvement. A moderate reduction in ventricular size was observed in 17% of the patients in our cohort. 40% of these patients showed good to excellent clinical improvement, 20% satisfactory improvement, and 40% unsatisfactory improvement. A marked reduction in ventricular size was observed in 6% of our patients. Of these latter patients, 50% showed good to excellent outcomes, while 50% had unsatisfactory outcomes. The favorable outcomes following implantation of a hydrostatic shunt in patients with normal pressure hydrocephalus did not correlate with decreased ventricular volume one year after operation. Better clinical outcomes were observed in patients with little or no alteration in ventricular size than in patients with a marked decrease in ventricular size. Postoperative change in ventricular volume should be assessed differently for patients with normal pressure hydrocephalus than in patients with hypertensive hydrocephalus.