• Change in sleep parameters, especially apnea-hypopnea index (AHI).
  

• Change in daytime sleepiness, assessed by Epworth Sleepiness Scale.
  
• Change in quality of life, assessed by SF-36.
  

Obstructive sleep apnea (OSA) is far the most common sleep disordered breathing, affecting 2-4% of the adult population. The repetitive obstructions are located in the pharyngeal airway, leading to sleep fragmentation and resulting in excessive daytime sleepiness with consequences for ability to work, road safety and quality of life. Furthermore, OSA is an independant riskfactor for cardiovascular disease. The treatment of choice today is continuous positive airway pressure (CPAP) but a main problem with CPAP is an unsatisfactory compliance. An alternative conservative more user-friendly treatment could be oral appliances, intending to increase the pharyngeal airway directly by tongue retaining devices or indirectly by mandibular advancing devices. Though several randomized studies on oral appliances have come recent years, all giving some evidence for effect on OSA, they all had some shortcomings, such as using crossover design, small sample sizes, under-reporting of methods and data and lack of blinding.

In this study of a mandibular advancement device was used a parallel group design with an inactive device and no intervention as controls. Beside the effect on sleep, daytime sleepiness and quality of life, the study aimed to find objective factors to be used as predictors of the outcome.