In a prospective study on the effect of shunting, 22 patients diagnosed as suffering from normal pressure hydrocephalus (NPH) were investigated by means of temporary external lumbar drainage (ELD). Five patients had to be excluded from the study because of complications of ELD or definitive shunting. ELD correctly predicted the outcome of shunting in all of the remaining 17 patients. The value of external lumbar drainage in NPH is discussed on the basis of personal experience and data from the literature. It seems to be a safe and valuable tool for predicting the outcome of definitive shunting procedures.