Persons with intellectual disability have difficulty in cooperating with outpatient care, and many are referred for general anaesthesia. Intellectual disability has traditionally been a contraindication for conscious sedation. We evaluated the behavioural impact, effectiveness, and tolerance of sedation in this population using a fixed 50% nitrous oxide/oxygen mixture as a single agent. We used dental treatment as a model of outpatient care; 349 patients (192 males, 157 females; mean age 22y [SD 14]; range 3-81y) were recruited over a 12-month period at seven centres. Sedation was deemed successful if planned dental treatment was completed. Behaviour was scored with the modified Venham scale. Out of 605 sessions, 91.4% were successful. No serious adverse effects occurred. Minor adverse events (such as nausea) occurred in 10.1% of sessions. We conclude that the use of safe and effective conscious sedation may reduce the indications for general anaesthesia.