The therapeutic effect of a functional magnetic system on obstructive sleep apnea and the system's operating mechanism have not been examined. Two hypotheses are postulated: a functional magnetic system increases the size of the oral cavity airway passage, or it increases the pharyngeal space. Twenty-eight patients with mild-to-moderate obstructive sleep apnea were examined; 10 patients (9 men and 1 woman; aged, 50.5 +/- 2.6 years) met the study criteria. After baseline nocturnal polysomnography and daytime tiredness self-evaluation, a functional magnetic system was inserted. The functional magnetic system is a mandibular repositioning appliance that uses a pair of attractive magnets (Sm2Co17), placed opposite each other in the jaws, which results in an advancement-to-opening ratio of 1:2. After 8 weeks of functional magnetic system treatment, polysomnography, daytime tiredness, and nighttime snoring were evaluated, and cephalogram radiographs with and without the appliance were taken. It was found that the respiratory disturbance index decreased significantly; minimal oxygen saturation increased significantly, reaching a normal value; day time tiredness improved; snoring declined; the oral cavity anterior region increased significantly, and the pharyngeal airway passages did not change. Reduction in the respiratory disturbance index and enlargement of the anterior oral cavity area were highly and significantly correlated. In conclusion, the functional magnetic system is a reliable mandibular repositioning appliance that has no apparent adverse effects. A functional magnetic system operates by increasing the anterior region of the oral cavity, mainly vertically, with no change in the posterior oral cavity region and pharyngeal airway passages.