The prevalence of idiopathic normal-pressure hydrocephalus (NPH) in a community was investigated by retrospective analysis of data from a previous community-based study of 170 randomly selected elderly residents aged 65 years or older. Magnetic resonance (MR) images of the subjects were reviewed for the specific structural features of idiopathic NPH, i.e. ventricular enlargement and narrow cerebrospinal fluid (CSF) space at high convexity and high midline areas. The clinical features of idiopathic NPH, gait disturbance, urinary incontinence, and cognitive impairment, were evaluated on the basis of records of the subjects' neurological examinations, a health questionnaire, the Mini-Mental State Examination, and Clinical Dementia Rating. Thirteen of the 170 subjects showed lateral ventricular enlargement greater than 0.3 on Evans' index. Five subjects (2.9%) demonstrated both ventricular enlargement and narrow CSF space at the high convexity/midline. All five subjects with these MR imaging signs had cognitive impairment, one had gait disturbance, and one had urinary incontinence. The present study found 2.9% of community-dwelling elderly subjects showed radiological and clinical features consistent with idiopathic NPH.