In a prospective study, 95 patients with mild to moderate obstructive sleep apnoea (OSA) were randomised to receive either surgical treatment, uvulopalatopharyngoplasty, (4-6 patients) or treatment with a nocturnal dental appliance for mandibular advancement (49 patients). Of the 49 dental appliance patients, 37 completed the 12-month follow-up. The aim of this study was to evaluate the effects and adverse events of dental appliance treatment from a one-year perspective. Somnography was employed to measure treatment effects before and 12 months post-treatment. At the 12-month control, somnography was performed twice: the first time with the dental appliance and the second time without it. Adverse events were recorded 2 weeks and 3, 6, and 12 months after treatment was initiated. The patients used the dental appliance on average 6 nights/week. After 12 months of treatment, the apnoea, apnoea/hypopnoea, oxygen desaturation, and snoring indices decreased significantly. Ninety-five per cent of the patients reduced their apnoea index by > or = 50% and 78% of the patients were normalised following treatment. At the somnographic registration without the dental appliance, the values were found comparable to what they were before treatment. Mandibular mobility and occlusion were constant throughout the study. The adverse events resulting from using the dental appliance were relatively minor and infrequent, and no serious complications were observed except for two patients who reported pain from the temporomandibular joint. In conclusion, the dental appliance has been shown to be a valuable treatment method for mild to moderate OSA with few adverse events in the stomatognathic system or other complications.