OBJECTIVE To clarify the characteristics of neuropsychiatric symptoms in patients with idiopathic normal pressure hydrocephalus (iNPH). METHODS Neuropsychiatric symptoms of 64 iNPH patients with mild triad symptoms from three kinds of hospitals were evaluated with the Neuropsychiatric Inventory (NPI) and compared with 126 patients with Alzheimer's disease (AD). RESULTS The most frequently observed neuropsychiatric symptom in the iNPH patients was apathy followed by anxiety and aggression. No symptom was more prevalent or more severe in iNPH than in AD. The severity of cognitive impairment was correlated with both aberrant motor activity and apathy. CONCLUSIONS Neuropsychiatric symptoms were mild in patients with iNPH and apathy was the most prevalent symptom. The correlation between neuropsychiatric symptoms and cognitive impairment in iNPH appears to arise from a common pathology in the frontal lobe.