Our purpose was to investigate side effects of sleep apnea treatment by removable oral appliances (OA) that advance the mandible. In 22 patients suffering from obstructive sleep apnea (OSA), questionnaire evaluations, polysomnographies, cephalographies, and dental plaster casts were obtained before initiation of treatment with OA that fully covered both dental arches. Patients were reevaluated after 3 to 12 mo (questionnaires, polysomnographies) and 12 to 30 mo (questionnaires, cephalographies, plaster casts) during continuous treatment. Polysomnographies confirmed improved breathing by OA. All patients experienced persistent alleviation of symptoms after 12 to 30 mo and wished to continue treatment. Side effects were common but only mildly disturbing: mucosal dryness (86% of patients), tooth discomfort (59%), and hypersalivation (55%). Cephalographies revealed a decrease in the mean (+/- SE) upper incisors to maxillary plane angle from 102 +/- 2 degrees at baseline, to 101 +/- 2 degrees after 12 to 30 mo (p < 0.05). Overbite and overjet were also slightly (mean reduction < 1 mm) but significantly reduced. None of these side effects required discontinuation of treatment. OA are an effective therapy of obstructive sleep apnea. Mild side effects are common but rarely require intervention. Nevertheless, close follow-up during long-term therapy by OA is advisable in order to timely detect potentially relevant orthodontic changes.