The objective of this study was to evaluate the effect of 2 different degrees of mandibular advancement, 50% vs. 75% of maximum protrusive capacity, on somnographic variables after 1 year of dental appliance treatment in patients with mild to moderate obstructive sleep apnea (OSA). A further purpose was to compare the number of adverse events on the stomatognathic system. In a prospective study, 74 male patients were randomly allocated to receive a dental appliance with either 50% (38 patients) or 75% mandibular advancement (36 patients). After 1 year of treatment, 55 patients completed the follow-up. Somnography was performed to measure treatment effects before and 12 months post-treatment. The apnea, apnea/hypopnea, and oxygen desaturation indices decreased significantly in both groups after 1 year (P < 0.001); however, there were no differences between the groups. Normalization (apnea index < 5 and apnea/hypopnea index < 10) was observed in 79% in group 50 and in 73% in group 75. Few patients (< 5%) reported symptoms from the stomatognathic system except for headache (> once a week), which was reported in one-third of the patients. Headache was significantly more infrequent after 1 year of treatment in both groups (P < 0.001). No serious complications were observed except for 2 patients who reported a painful condition from the temporomandibular joint in either group. In conclusion, mandibular advancement with a dental appliance effectively reduces the sleep-breathing disorder measured as frequency of apneas, and a pronounced mandibular advancement did not show a greater improvement of the medical problem compared to less advancement for patients with mild to moderate OSA. On the basis of few adverse events in the stomatognathic system or other complications we can recommend dental appliance treatment and, for patients with mild to moderate obstructive sleep apnea, not starting treatment by more than 50% mandibular advancement.