Introduction: Idiopatic normal pressure hydrocephalus is a disease with unknownpathophysiology that comes with a great impact on the patient’s life. Hakim’s triadexplains the condition as gait impairment, dementia, and urinary incontinence. The onlytreatment for iNPH, which is a progressive disease, is shunt surgery. It is previouslyindicated that early intervention might be of importance for the outcome.Aim: To investigate if the time between decision to accept the patient for surgery andshunt surgery, negatively influences the outcome.Methods: Eligible for this study was all patients (n=3060) registered in The SwedishHydrocephalus Quality Registry (SHQR) during 15th of January 2004-24th of May 2019.Excluded was those who had an extreme waiting time defined as four years or longer.Outcome measured using the definition of improvement in iNPH score, and also time up andgo (steps and seconds) and mini mental state examination.Results: Results: At 3 months 57 % (p=0.0053) of the patients with less than 3 monthswaiting time showed an improvement in iNPH score (≥5 points). A waiting time of morethan 3 months and more than 6 months had the same improvement in 50 % and 47 % ofthe patients, respectively. At 12 months follow-up the same improvement of iNPH scorewere respectively 61 %, 52 % (p=0.0214) and 52 % (p=0.0396) for the three waiting timegroups.Conclusions: This population-based study showed that shunt surgery should be performedwithin 3 months of decision to surgery, to attain the best outcome.