Obstructive Sleep Apnoea (OSA) is a sleep disorder characterised by repetitive upper airway collapse during sleep leading to nocturnal oxygen desaturation and sleep fragmentation. One treatment is a Mandibular Advancement Splint (MAS), a dental device which holds the lower jaw forward during sleep and increases upper airway space. However treatment success in reducing upper airway collapse varies. Collapsing pressures on the upper airway are thought to result from excess soft tissues surrounding the airway compared to size of the craniofacial bony enclosure, or ‘anatomical imbalance’. MAS treatment increases size of the lower face enclosure. MAS effectiveness in correcting ‘balance’ of upper airway anatomy may depend on size of the soft tissues in a given patient. We hypothesise OSA patients with anatomical imbalance may not respond to therapy as MAS is insufficient to overcome the excess tissue. We aimed to assess 1) whether anatomical balance differs between MAS treatment responders and non-responders and 2) whether alteration of anatomical balance with MAS is related to treatment outcome. Method: 69 OSA patients had upper airway MRI without and with MAS in situ. Image analysis was performed to obtain an upper airway soft tissue (ST) volume and bony maxillomandibular enclosure volume (MEV). MEV was assessed without and with MAS. Anatomical balance was calculated as ST/MEV volume ratio. Results: There was no difference in ST/MEV between Responders (0.96 ± 0.02, n=36) and Non-Responders (0.98 ± 0.02, n=33). With MAS, airway volume increased (10.6±4.7 vs. 11.5±48 cm³, p<0.05), ST volume did not change, MEV volume increased (309.3±43.3 vs. 324.5±43.8 cm³, p<0.001). ∆ST/MEV with MAS did not differ between responders and non-responders (-0.03±0.02 vs. -0.03±0.03). Conclusion: Anatomical balance, calculated as ST/MEV ratio, did not show an association with MAS treatment response. Further work is required to understand the mechanisms behind the inter-individual response to MAS.