Treatment by using a mandibular advancing positioner (MAP) was markedly effective in two cases of severe obstructive sleep apnea–hypopnea syndrome (OSAHS). Two patients complaining of snoring, apnea, and daytime sleepiness were examined by the use of a polysomnography and diagnosed to have severe OSAHS. Facial cephalometry and profilography showed marked mandibular retraction, microgenia, hypertrophy of the lingual tonsil, and marked airway stenosis at the root of the tongue. On the snoring sound test, snoring disappeared when the mandible was put in the protrusive position. Therefore, MAP was applied, and marked improvements were observed. From these results, MAP is considered to be effective for the treatment of severe OSAHS when snoring disappears in the protrusive position on the snoring sound test.