The aim of the present study was to evaluate whether the outcome of treatment using an intraoral mandibular advancement device in patients with obstructive sleep apnoea is associated with the mandibular morphology. The effects of the device on apnoeas and sleep were evaluated in 32 men with obstructive sleep apnoea in continuous polysomnographic sleep recordings including body position, during one night without the device and one night with it. Mandibular morphology variables were measured on cephalograms. The odds ratio for a supine apnoea-hypopnoea index of below 15 during treatment was 17 for a mandibular plane angle of 38 degrees or below, and 26 for a lower anterior face height of less then 73 mm. The outcome of treatment in the lateral sleep position was unrelated to any mandibular morphology variable. Patients with supine-dependent sleep apnoea defined by a supine apnoea-hypopnoea index of 10 or above and a lateral apnoea-hypopnoea index of below 10 had an odds ratio of 7 to have an orthognathic mandible with an SNB angle of 78 degrees or above. The present study suggests that a successful apnoea reduction using a mandibular advancement device is associated with a normal mandibular plane angle and a small lower anterior face height.