INTRODUCTION: Continuous positive airways pressure, generally used to treat obstructive sleep apnea-hypopnea syndrome (OSAHS), is not tolerated well by many patients. An alternative is to treat OSAHS with mandibular advancement devices (MAD). This research assesses the long term (> 2 years) effectiveness and the side effects on dental and skeletal parameters of these devices. MATERIAL AND METHODS: We selected 24 patients with moderate to severe OSAHS. All were treated with MADs for at least 2 years. We gathered cephalometric teleradiographs in centric relation and sleep recordings before and after the patients were treated. We evaluated the patients' apnea-hypopnea indexes (AHI) as well as their Epworth sleepiness scale (ESS) scores. We measured the inclination of the central incisors and the positions of the upper and lower jaws. RESULTS: Mandibular advancement devices were used for more than 2 years (3.9 ± 1.9 years). We observed a statistically significant decrease of the patients' AHI and their ESS scores. We also observed a modification of the inclination of the lower central incisors (+0.521; P = 0.047) and of the position of the maxilla (-0.287; P = 0.039). DISCUSSION: We demonstrated the clinical effectiveness of mandibular advancement devices for treating OSAHS, with a very low rate of side effects on dental and skeletal positions.