The surgical results obtained with a cerebrospinal fluid (CSF) shunt operation in 13 patients considered to be suffering from normal pressure hydrocephalus have been correlated with the findings of the constant manometric infusion test (IT) and of prolonged intracranial pressure recordings. A positive IT, high amplitude CSF pulse pressure and large transitory increases of CSF pressure during sleep seem to be useful criteria for the surgical prognosis. Ten more patients affected by primary cerebral atrophy have also been studied. The data obtained in both groups of patients have been utilized for a possible pathogenetic interpretation of the syndrome.