C.S.F. resistance to flow (Rf) was measured in 24 patients by the mean of a constant infusion manometric test, and the results were compared with those of pneumo-encephalography and isotope cisternography. It is concluded that Rf is constantly elevated in patients with normal pressure hydrocephalus, is at least as reliable as the two other tests and therefore provides a really valuable aid to the identification of the patients to be shunted.