Background/Aims: We developed an idiopathic normal-pressure hydrocephalus grading scale (iNPHGS) to classify a triad of disorders (cognitive impairment, gait disturbance and urinary disturbance) of iNPH with a wide range of severity. The purpose of this study was to assess the reliability and validity of this scale in 38 patients with iNPH. Results: The interrater reliability of this scale was high. The iNPHGS cognitive domain score significantly correlated with the cognitive test scores, including the Mini-Mental State Examination (MMSE), the gait domain score with the Up and Go Test and Gait Status Scale scores, and the urinary domain score with the International Consultation on Incontinence Questionnaire-Short Form (ICIQ-SF) score. The MMSE, Gait Status Scale and ICIQ-SF scores significantly improved in patients whose iNPHGS scores improved after CSF tapping but not in those whose iNPHGS scores did not improve after CSF tapping. Fourteen of the 38 patients received shunt operations. In these 14 patients, changes in the iNPHGS cognitive and urinary domains after CSF tapping were significantly associated with the changes after the shunt operation.