Twenty Down's syndrome dental patients between 9 and 20 years of age have been identified in order to evaluate anxiety and pain induced by various kinds of dental treatment such as some "moderately painful" dental out-patient procedures (10 subjects), general anaesthesia dental treatment (5 subjects) and painless out-patient visit or routine orthodontic services (5 subjects). Each patient received a previous psychological evaluation to assess his intellectual and cognitive developmental degree. Anxiety was assessed twice, before and after intervention by means of a questionnaire; pain was tested in a subjective way during the procedure (or the control oral visit 12 hours after the g.a. treatment) and by a questionnaire in the post-treatment phase. This investigation indicates that anxiety increased in Down's syndrome subjects receiving out-patient interventions, remained at the original level in the general anaesthesia hospitalized patients, while it showed a clear reduction in the out-patient painless control group. Anxiety, however, proves to be influenced also by the patient's effective mental age so it my be lessened after painful dental treatment when the intellectual development is about or above that of a 7 year old boy. Our study could prove to be important since it underlines the need to assess, at least approximately, the Down's syndrome patient's cognitive development before giving dental treatment, especially if it is an out-patient intervention. This may not be necessary for routine free of pain dental visits.