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Introduction Idiopathic normal pressure hydrocephalus (iNPH) is a disease that comes with a great impact on the patient’s life. The only treatment for iNPH, which is a progressive disease, is shunt surgery. It is previously indicated that early intervention might be of importance for the outcome. Aim To investigate if a longer waiting time for surgery, negatively influences the clinical outcome. Methods Eligible for this study were all iNPH patients (n = 3007) registered in the Swedish Hydrocephalus Quality Registry (SHQR) during 1st of January 2004–12th of June 2019. Waiting time, defined as time between the decision to accept a patient for surgery and shunt surgery, was divided into the intervals ≤ 3, 3.1–5.9 and ≥ 6 months. Clinical outcome was assessed 3 and 12 months after surgery using the modified iNPH scale, the Timed Up and Go (TUG) test and the mini mental state examination (MMSE). Results Three months after surgery, 57% of the patients with ≤ 3 months waiting time showed an improvement in modified iNPH scale (≥ 5 points) whereas 52% and 46% of patients with 3.1–5.9 and ≥ 6 months waiting time respectively improved (p = 0.0115). At 12 months of follow-up, the corresponding numbers were 61%, 52% and 51% respectively (p = 0.0536). Conclusions This population-based study showed that in patients with iNPH, shunt surgery should be performed within 3 months of decision to surgery, to attain the best outcome.