Mentally retarded children from one age cohort and their randomly selected controls in one Finnish county were examined for standard of oral hygiene, and their parents and nurses interviewed for information on dental health habits and for other relevant background information. The mentally retarded consumed sugar containing products less frequently than the mentally normal controls. Toothbrushing and fluoride supply was also less common in the retarded than in the healthy. Among the retarded, the dental health habits were most favorable in registered outpatients and least favorable in administratively unknown retarded not included in special welfare. Differences in dental health habits between these subgroups of the retarded were large. In contrast to the findings in the healthy children, frequency of toothbrushing was not associated with the observed standard of oral hygiene in the mentally retarded. The toothbrushing subgroup of the mentally retarded consumed sugar more frequently than the toothbrushing healthy children and commonly used drugs which reduced saliva flow or promoted gingival hyperplasia.