The obstructive sleep apnea syndrome is characterised by repeatedly occurring complete or partial obstructions of the upper airway during sleep, which can be accompanied by serious oxygen desaturations. This can result in cardiovascular co-morbidity and excessive daytime sleepiness, with an increased chance of motor vehicle accidents and diminished performance at work. The use of a mandibular advancement device appears to be an effective therapy. In the long term, however, the possibility of dental side effects should be taken into consideration. Development of a relative mesio-occlusion has frequently been observed. Side effects are usually mild and transient. To objectively evaluate whether the side effects are stable or progressive, a thorough follow-up is needed. It is therefore desirable that treatments with a mandibular advancement device are carried out by dentists or specialists with experience and special expertise in this area.