An evaluation of three-year incremental caries scores in institutionalized persons with various mental disorders indicated that in a DMFS evaluation of caries increment, the group with Down syndrome had significantly fewer caries than many of the other groups compared. No such significant difference was found in an evaluation of DMFT. The caries increments did not differ significantly from one another in all other mentally subnormal groups. The carious lesions in the group with Down syndrome appeared to be limited to the occlusal surface.