Rationale: Predicting which patients with obstructive sleep apnea (OSA) will be successfully treated with mandibular advancement splints (MAS) remains elusive. Developing simple daytime measurements and tests to predict treatment outcome would enhance MAS treatment. Objective: The purpose of this study was to assess the clinical utility of anthropomorphic measurements and cephalometric X-rays in the prediction of MAS treatment outcome in OSA. Methods: Anthropomorphic measurements and cephalometric X-rays from 72 OSA patients who had presented to a tertiary referral sleep clinic were analyzed retrospectively. Results Treatment response was defined as ≥50% reduction in Apnea/Hypopnea Index (AHI; criterion 1); ≥50% reduction and residual AHI less than 20/h (criterion 2); ≥50% reduction in AHI and residual AHI less than 10/h (criterion 3); and ≥50% reduction in AHI and residual AHI less than 5/h (criterion 4). This was done to reflect the differences in the clinical definition of treatment success in the literature. A good response occurred in 56% (40 patients) according to criterion 1; 54%(39 patients) according to criterion 2; 46% (33 patients) according to criterion 3; or 39% (28 patients) according to criterion 4. Age and gender were found to be significant predictors for criteria 1 and 2. Age and soft palate length were found to be significant predictors forcriteria 3 and 4. Equations to predict MAS treatment response were derived as equations were to predict final AHI. Conclusions: Certain cephalometric and anthropomorphic measurements impact on MAS treatment outcome. This study adds to the current literature and implies that MAS success is (to some degree) related to anatomical characteristics.