Epidemiological analyses, with respect to periodontal disease severity according to signs and symptoms using the Ramfjord Calculus Index (CI) and Periodontal Disease Index (PDI), were performed on 881 mental retardates possessing one or more subject teeth at 15-48 years of age. They were among a group of 1107 subjects who were admitted to the Niigata Prefectural Institute of Total Training for the Mentally Retarded, Colony Niigata Shiroiwa-no-Sato, during the years 1971-1988. The average number of sextants without signs and symptoms of disease decreased, and that of gingivitis, calculus accummulation, and periodontitis increased with age in 3 periods of admission, 1971-1975, 1976-1981 and 1982-1988. After the mid 1970's, the time and sex appeared to have no influence, but these two factors did seem to have some influence in the first half of the 1970's. IQ and the experience of institutionalization had no influence. Average number of sextants with gingivitis and periodontitis were considered similar and higher, respectively, than those of the standard Japanese population.