The aims of this study were to investigate possible orthodontic side-effects following the use of mandibular advancement devices (MAD) in adults with snoring and sleep apnoea. A second objective was to analyse the effect of the appliance design. Seventy-five patients treated with MAD and 17 reference patients were studied at follow-up after 2.5 +/- 0.5 years. In the test group, 47 patients were provided with soft elastomeric devices, while the remaining 28 patients received hard acrylic devices. The treatment induced a change in overjet of -0.4 +/- 0.8 mm (mean +/- SD) and a change in overbite of -0.4 +/- 0.7 mm (mean +/- SD). These changes were larger than those found in the reference group (P < 0.01). The odds ratio (OR) for the largest quartile of reduction in overjet was 3.8 in patients using hard acrylic devices compared with those using soft elastomeric devices (P < 0.05). A large reduction in overjet in patients using the hard acrylic devices was unrelated to the degree of mandibular protrusion by the device. The OR for a large reduction in overjet in patients using the soft elastomeric devices with a protrusion of 6 mm or above was 6.8 compared with smaller mandibular protrusions (P < 0.05). The results indicate that the orthodontic side-effects are small during the treatment of adult subjects with MAD for snoring and sleep apnoea, especially in patients using soft elastomeric devices with mandibular protrusions of less than 6 mm. The follow-up of patients treated with MAD is recommended, as individual patients may experience marked orthodontic side-effects.