Purpose: Reports of irreversible alteration in jaw posture and destructive occlusal contact relationships in individuals using mandibular advancement devices for obstructive sleep apnea are beginning to appear. This study sought cephalometric means of identifying such individuals before commencing therapy. Materials and Methods: Cephalograms of 34 obstructive sleep apnea sufferers who had worn mandibular advancement devices for 2 years were compared retrospectively with baseline films taken at commencement of therapy and analyzed for signs ofmorphologic changes in jaw position and occlusal relationship. In affected patients, two distinct morphologic species of mandibular reposturing became evident: (1)bilateral posterior open bite with destructive incisal attrition; and (2)less destructive intermediate open bite over the premolar and first molar regions. From the observed morphology patterns, gonial angle and maxillary-mandibular plane angle were analyzed as possible vertical cephalometric risk predictors, with newly defined pterygoid advancement proportion (PtAP) as a horizontal predictor. Results: Three patients displayed the posterior open bite pattern and had gonial angles = 119 degrees and maxillarymandibular plane angles = 16 degrees, with PtAP values >= 0.48. Prediction intervals for the five intermediate open bite cases were 118 degrees = gonial angle = 128 degrees, and 23 degrees = maxillary-mandibular plane angle = 32 degrees. PtAP values were >= 0.52. Conclusion: Cephalometric analysis can help practitioners identify which apnea patients might be likely to develop irreversible mandibular postural changes from wearing a jaw-repositioning device.