The aims of this retrospective study were to assess the effect of a Karwetzky mandibular protrusion appliance for treating patients with mild, moderate, and severe obstructive sleep apnea. Eighty‐one of 116 patients (69.8%) suffering from obstructive sleep apnea were treated with an activator model according to Karwetzky. After 4 months (SD 4.0 months) treatment outcome was controlled by polysomnography. Therapeutic outcome depended on the severity of obstructive sleep apnea. The median apnea‐hypopnea index decreased from 10.6 events/h (range 2.0–14.9) to 5.8 events/h (range 0.2–17.3, P<0.01) in the mild group, from 21.7 events/h (range 17.3–28.4) to 7.7 events/h (range 1.0–30.1, P<0.001) in the moderate group, and from 42.1 events/h (range 33.2–64.9) to 18.1 events/h (range 2.4–48.8, P<0.001) in the severe group. Sleep variables did not show consistent improvement except for a trend towards more REM sleep and slow‐wave sleep. The numbers of retentive teeth did not statistically influence treatment efficacy. Comparing the pre‐ and post‐treatment polysomnographic variables, it was found that the respiratory events rather than sleep stages were significantly reduced by the Karwetzky appliance investigated. [ABSTRACT FROM AUTHOR]