A dental orthosis, designed to increase the size of the upper airway by advancing the mandible, was used in 68 patients to treat snoring and varying degrees of obstructive sleep apnea. The orthosis increased the posterior airway space, as assessed by lateral cephalograms. Seven months (range 2 to 25) after beginning use, 75 percent of patients were using the orthosis regularly. Snoring, by report, was improved in all patients but one (p less than 0.001) and was eliminated in 42 percent (95 percent confidence interval 30 to 55 percent). Sleep quality and sleepiness were also reported improved. Apneas and hypopneas, measured before and after use in 20 patients with obstructive sleep apnea, were reduced from an average of 47 to 20 events per hour (p less than 0.001). Oxygenation and sleep disturbance were also improved. Apnea-hypopnea frequency was reduced with treatment to less than 20/h in 13 patients. Residual frequencies greater than 20/h were associated with higher initial frequencies of apneas and hypopneas. Side effects of orthosis use were minor, and no serious complications were observed. The dental orthosis is an effective treatment for the symptom of snoring and can also effectively treat obstructive sleep apnea of moderate severity.