Mandibular advancement devices (MAD) are recommended in certain cases for the treatment of obstructive sleep apnea and hypopnea syndrome (OSAHS). The aim of this study was to make a retrospective investigation into the dental and skeletal effects in patients suffering from OSAHS after wearing an MAD, and according to the type of device, rigid or semi-rigid. Cephalometric criteria on lateral cephalograms before and after at least 6 months of treatment were analyzed. Twenty-two patients were included. We observed average labial tipping of the lower incisors (+4.1 ± 5.6 [P = 0.0023]) and reduced overbite (−1.6 ± 2.0 mm [P = 0.0026]) and overjet (−1.4 ± 2.3 mm [P = 0.0114]). Labial tipping of the mandibular incisors was more pronounced with semi-rigid MAD. Similarly, the decrease in overbite and overjet was more marked with the semi-rigid MAD. Medium-term wearing of MAD gives rise to dentally related occlusal changes, which can only be corrected by orthodontic treatment.