The purpose of this Study was to examine the efficacy and compliance of a mandibular advancement device(MAD) according to the severity of sleep apnea in the snorers and obstructive sleep apnea patients. Fifty-four patients (45 males, 9 females, aged 20 - 68years ) who visited Seoul National Uiversity Dental Hospital(SNUDH) to seek for the treatment of snoring and sleep apnea were classified into four groups according to the results of the nocturnal polysomnography and they were instructed to wear MAD regularly which was designed to increase the size of the upper airway by advancing the mandible. The evaluation of the efficacy and compliance of the MAD according to the severity of apnea and the duration after the usage of MAD ( 1week, 1month, 3months, 6months, 12months) was made by using quesionnaires mad in Department of Oral Medicine and Oral diagnosis, SNUDH. The obtained results were as follows : 1. All subjects results were habitual snoreres and 43 patients(79.6%) complained the loudness of snoring that can be heard out of the room. 2. Apnea index(AI) of the total subjects was mean 29.4$pm$26.9 and respiratory disturbance index(RDI)was mean 37.6$pm$28.0. And there was nodifference in the efficacy and the compliances of MAD according to the severity of apnea. 3. The severityi of apnea by the questionnaires significantly corresponded with the results of nocturnal polysomnography, and this fact potentiated the diagnostic value of the questionnaire. 4. after the usage of MAD, there was significant improvement in the frequency of snoring, the loudness of snoring, frequency of apnea, daytime sleepiness nad the refreshment after sleep(p<0.001) regardless of the apnea index(AI) and respiratory distrubance index(RDI). 5. The degree of the satisfaction with MAD was mean 74.4$pm$18.4% and that of the discomfort with the MAD was 31.4$pm$19.6%. But there was no serious complication in occlusion and temporomandibular joint with the usage of MAD and the duration of the discomfort was mean 3.3$pm$2.2 weeks. 6. Forty-one patients(75.9%) continued the usage of MAD but thirteen patients(24.1%) stopped the use of MAD because of the discomforts and insufficient results with it.