The objective of this study was to compare the effect of two different degrees of mandibular advancement (MA), 75% versus 50%, on somnographic variables after 6 months of dental appliance treatment in patients with severe obstructive sleep apnea (OSA). A further purpose was to compare the number of adverse events on the stomatognathic system and the effects of dental appliance treatment on the presence of daytime sleepiness. Eighty-six males with severe OSA (apnea index > or = 20) were randomly allocated to either 75% or 50% MA. Forty patients in the 75% MA group and 37 patients in the 50% MA group completed the 6-month follow-up. The effectiveness of treatment in terms of normalization (apnea index < 5 and apnea/hypopnea index < 10) with 75% MA was 52%, which was significantly higher (p = 0.04) than the 31% achieved with 50% MA. The dental appliance had few adverse events on the stomatognathic system regardless of group, and the number of adverse events did not differ between the two groups. Finally, the mean value of Epworth Sleepiness Scale scores decreased significantly from 11.6 at baseline to 8.0 at follow-up (p < 0.001). No significant difference was observed between the two groups. The results indicate that a dental appliance could be an alternative treatment for some patients with severe OSA.