One hundred and eighty-nine mentally handicapped persons attending both residential and non-residential institutions were examined during 1984/85. All were assessed for treatment need for caries; periodontal need was also assessed in 43 of the older patients. At base-line examination it was found that a high proportion of the dental caries remained untreated and most of the treatment that was provided consisted of extractions. Treatment was subsequently provided by a health board dental officer, junior hospital staff, final year undergraduate students or a consultant in paediatric dentistry. Most treatment requirements were met by the non-consultant personnel. A follow-up examination completed during 1987 showed a considerable improvement in the treatment of caries; periodontal treatment need remained unchanged. It can concluded from this study that it is feasible to set up an effective dental service for mentally handicapped groups, that improvements in the treatment of dental caries can be achieved in a relatively short period of time and that most treatment requirements can be met by non-consultant staff. Continuing care, particularly for periodontal disease, remains a problem.