In order to investigate the morphological characteristics of Japanese patients with obstructive sleep apnea (OSA) and basis for selection of an oral appliance (OA) therapy, the craniofacial skeleton, soft tissue and upper airway were analyzed on lateral cephalograms from 103 patients with OSA and 98 control subjects. At first, the 103 OSA patients were divided into two groups according to their body mass index (BMI). The results were the following: Micrognathia was a major risk factor of Japanese OSA patients instead of obesity; the narrowing in the hypopharynx was observed due to the inferior-located hyoid bone, especially in the obese group, an enlarged tongue and sticking of fatty-tissue to upper airway wall were observed in addition. Secondly, the 103 OSA patients were treated by OA, and were classified according to the reduction ratio of AHI (rAHI) again. The results suggested that the patients group which showed rAHI > or = 50% had a balanced relationship between the maxilla and mandible. These findings proved that the morphological characteristics specific to Japanese patients with OSA and patients who were allowed to accept the OA therapy were exist.