OBJECTIVES: Severe dental malocclusion resulting from mandibular advancement devices (MADs) for obstructive sleep apnea syndrome (OSAS) is extremely rare. Reporting of such situations is therefore useful for clinicians managing OSAS. STUDY DESIGN: We report the unusual cases of 4 patients who developed a severe circular dental open bite related to the use of a MAD. These bite changes were related to molar extrusion in two patients, whereas in the other two patients no obvious etiology was found. RESULTS: In all patients, the MAD was immediately discontinued as early as the malocclusion was detected and continuous positive airway pressure proposed. CONCLUSION: Mandibular advancement devices may be associated with a severe dental malocclusion. Failure to recognize and promptly treat this complication may lead to irreversible dentoskeletal changes. Therefore, follow-up to survey dental occlusion is mandatory in all patients wearing a MAD.