The purpose of this study was to estimate the prevalence of mental retardation (MR) in Korea. The study population comprised 1,757 third-grade elementary school children residing in a single school district in Suwon, Korea. We conducted the total population survey for 1,537 children who attended one of 5 schools in the district. For the remaining 220 children who did not attend a school in the school district, we found children with MR via inspection of the register list of the disabled. A total of 16 children (7 of the 1,537 children and 9 of the 220 children) were found to have MR, yielding a prevalence of MR of 9.1/1,000. Considering that about 60% of the people with disabilities are registered in Korea, there would be at least 6 unregistered children with MR among the 220 children, so that the prevalence of MR can be estimated at 12.5/1,000 for the Suwon area.