We investigated if there are individuals at a preclinical stage of idiopathic normal pressure hydrocephalus (iNPH) in a general population. All the residents aged 61 years and 70-72 years in the two communities of Japan (n=1142) were requested to take brain MRI examination. 790 residents (69.2%) participated in this study. Among them, 51 individuals (6.46%) had the enlarged ventricles (Evans index of >0.3), 12 (1.52%) of which showed the iNPH features on MRI. Of the 12 individuals, 8 were asymptomatic, while 4 had gait disturbance and/or dementia (possible iNPH). During a follow-up period of 4-8 years, two of the 8 asymptomatic subjects developed dementia and/or gait disturbance. The prevalence of possible iNPH was 0.51% (4/790) among Japanese elderly (>61 years of age). Asymptomatic ventriculomegaly with the iNPH features on MRI (AVIM) may represent a preclinical stage of iNPH. To identify a possible genetic change of AVIM and possible iNPH, we performed a genome-wide screening for copy number variations (CNV), and found that a segmental copy number loss of the SFMBT1 gene may be a genetic risk for the disease.