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The natural course of idiopathic normal pressure hydrocephalus (iNPH) has not been thoroughly studied. The consequences of postponing shunt treatment are largely unknown. We aimed to describe the effects of waiting for more than 6 months before surgery and to compare the outcome with that seen in patients who waited for less than 3 months.33 patients (iNPHDelayed) underwent an initial investigation (Pre-op 1), followed by re-examination, just prior to surgery, after waiting for at least 6 months (Pre-op 2). Outcome was evaluated after 3 months of treatment. 69 patients who were surgically treated within 3 months after Pre-op 1 and who were also evaluated after 3 months of treatment constituted a comparison group (iNPHEarly). Evaluations were done with the iNPH scale and the modified Rankin Scale (mRS). iNPHDelayed patients were prospectively studied with regard to outcome, whereas the comparison group iNPHEarly was defined and analysed retrospectively.iNPHDelayed patients deteriorated significantly during their wait for surgery, with progression of symptom severity ranging from +7 to −47 on the iNPH scale, and from 0 to +3 on the mRS (both p<0.001). The magnitude of change after surgery was similar in the groups, but since the symptoms of iNPHDelayed patients had worsened while waiting, their final outcome was significantly poorer.The natural course of iNPH is symptom progression over time, with worsening in gait, balance and cognitive symptoms. This deterioration is only partially reversible. To maximise the benefits of shunt treatment, surgery should be performed soon after diagnosis.