We studied retrospectively the effectiveness of the repeated lumbar CSF tap test (RTT), lumbar external CSF drainage (LED) and radioisotope cisternography (RIC) in predicting the outcome of shunt surgery, as well as the diagnostic and prognostic value of periventricular hyperintensity (PVH) and of the classic clinical triad in normal pressure hydrocephalus. Two hundred and seventy patients were referred to the Departments of Neurosurgery, in Nancy, France and in Istanbul, Turkey. The decision to perform surgery was based on the clinical presentation (all patients had at least two symptoms of the classic clinical triad), neuroimaging examinations and the results of the RTT (taps were performed on three consecutive days and at each tap a minimum of 30 to 40 cc of CSF was removed), the LED (drainage was performed for 3 days and the volume of CSF drained daily was a minimum of 150 to 250 cc) or the RIC. After all shunt procedures, postoperative assessments verified improvements in 88% of the RTT group, 91% of the LED group and 66% of the RIC group. Gait disturbance had improved in 90% at the end of the second and twelfth month follow-up. Cognitive dysfunction had improved in 79% at the second and in 77% at the twelfth month follow-up. Urinary incontinence had improved in 66% at the second and in 62% at the twelfth month follow-up. From the surgical point of view, the greatest difficulty is not to make the diagnosis, but rather to identify the appropriate patients to operate on. The decision to perform shunt surgery should be based on strict clinical findings associated with CT and MRI criteria and especially with positive RTT or LED test results.