Oral hygiene, periodontal health and periodontal treatment needs were studied in the dentulous patients (n = 328) of a sample selected to be representative of all institutionalized mentally subnormal persons in Norway aged 5-45. The average oral hygiene and periodontal health was poor except for patients in a few institutions where the nurses were trained to clean teeth regularly. Increased age, epilepsy, Down's syndrome and a high degree of mental deficiency were all elements that apparently contributed to impairment of periodontal health and to increased treatment requirements. The preventive programs used in some institutions seemed to be effective. Still better results, however, could probably be obtained by the use of dental hygienists.