Background: Idiopathic normal pressure hydrocephalus (iNPH) is frequently associated with concomitant amyloid-β (Aβ) pathology. Objective: To compare the [ 11 C]PIB PET uptake in the patients with suspected iNPH to Aβ and hyperphosphorylated-tau (HPτ) in the right frontal cortical biopsy, the cerebrospinal fluid (CSF) Aβ, the response to a CSF shunt, and the final clinical diagnosis of Alzheimer's disease (AD). Methods: Patients (n = 21) from Kuopio NPH Registry (http://www.uef.fi/nph) with intraventricular pressure monitoring, immunostaining for Aβ and HPτ in the right frontal cortical biopsies, and a Mini-Mental State Examination and a Clinical Dementia Rating underwent [ 11 C]PIB PET. Aβ, total tau, and Pτ 181 were measured by ELISA from the ventricular (n = 15) and the lumbar (n = 9) CSF. Response to the shunt was seen in 13 out of the 15 shunted patients. AD was diagnosed in 8 patients during a median follow-up of 6 years (mean 7.3±2.4 years, range 3-1). Results: [ 11 C]PIB uptake in the right frontal cortex (ρ= 0.60, p < 0.01) and the combined neocortical [ 11 C]PIB uptake score (ρ= 0.61, p < 0.01) were associated with a higher Aβ load in the right frontal cortical biopsy. Excluding one (1/15) outlier, [ 11 C]PIB uptake was also associated with the ventricular CSF Aβ (ρ= -0.58, p = 0.03). Conclusions: The findings show that [ 11 C]PIB PET can reliably detect simultaneous amyloid pathology among the iNPH patients. Further studies will show whether amyloid PET could predict a clinical response to the shunt operation. In addition, the presence of Aβ pathology in the patients with iNPH might also warrant treatment with current AD drugs.