Purpose: Mandibular Advancement Devices (MAD) are a common treatment option in patients with Obstructive Sleep Apnea (OSA) and Continuous Positive Airway Pressure (CPAP) intolerance. The aim of the present study is to examine the efficacy of the Somnodent®-flex MAD in a cohort selected by pretherapeutic Drug-Induced Sleep Endoscopy (DISE). Methods: In a retrospective cohort study, 135 patients were examined with Somnodent®-flex. Apnea-Hypopnea Index (AHI) and Oxygen-Desaturation Index (ODI) was measured with respiratory polygraphy before therapy and 12 months after using MAD in 112 patients. Daytime sleepiness using the Epworth Sleepiness Scale (ESS), Fatigue Severity Score (FSS) and snoring index (0-10), were assessed by means of a questionnaire in 94 patients. The consequent side effects were then documented. Results: After having used MAD, the AHI was reduced by 39.6% (-9.4/h) from 23.8 ± 15.9/h to 14.4 ± 12.5/h, the ODI was reduced by 37.6% (-7.1/h) from 18.9 ± 16.2/h to 11.8 ± 11.0/h, the mean ESS by 4.4 from 7.5 ± 5.0 to 3.2 ± 2.9 and the FSS by 10.4 from 30.4 ± 16.2 to 20.0 ± 14.1. Snoring was reduced by 5.3 from 8.2 ± 1.8 to 2.9 ± 1.8 on a VAS. The study found all noted changes to be significant. Furthermore, a mean usage time of 6.3 hours was reported with 82.8% daily use after 12 months. A low rate of registered side effects in long-term MAD usage were observed. Conclusion: The Somnodent®-flex MAD is an effective treatment option for therapy of OSA. However, patient selection using pre-therapeutic DrugInduced Sleep Endoscopy (DISE) showed similar results as reported in the literature for unselected groups of patient