Mandibular advancement devices (MAD) should be contraindicated (Clark GT, Sleep Med Rev 1998;2:163-174) if there are: (1) insufficient teeth to support the device, (2) periodontal problems inducing tooth mobility, (3) active temporomandibular joint (TMJ) disorder, and (4) limited maximum protrusive distance (< 6 mm). The aim of the present study was to evaluate the proportion of the obstructive sleep apnea population that exhibits any contraindication (CI) to MAD. For this study there were 100 unselected adult patients consecutively diagnosed by polysomnography in a tertiary sleep laboratory. Clinical and radiologic evaluation of the dental, periodontal, and TMJ status of these 100 patients were performed by two expert maxillofacial surgeons, blind to each other, permitting the identification of MAD CIs. The two maxillofacial surgeons agreed on MAD absolute CIs in 96 of the 100 patients. CIs were identified in 34% of the patients. The nature of the CIs systematically referred to an insufficient number of remaining teeth (mean number of teeth lost: 7.8 +/- 6.1 with 31 patients having had more than 10 teeth removed). The tooth avulsions were significantly higher in contraindicated compared with noncontraindicated patients (16 +/- 8 versus 4 +/- 3, p < 0.00001). Periodontal abnormalities coexisted with dental CI in approximately half of the patients. A TMJ disorder was considered as significant enough to lead to CI in two patients. Dental and periodontal care was needed in 16 patients before the use of MAD could be considered. Conclusions were that primary CIs were present in 34% of 100 consecutive patients, mainly owing to dental problems. Moreover, another subgroup of patients (16%) required close supervision and follow-up to avoid impairment of preexisting TMJ and dental problems. Such a high rate of CI should be considered when the overall efficacy of oral appliances is compared with other treatments, such as surgery or nasal continuous positive airway pressure.