The mophologic study of adults with true Class III malocclusion has seldom been reported. Tracings of lateral cephalograms of 51 Taiwanese adults with true Class III malocclusion were subjected to quadrilateral analysis. Means and standard deviations, + test, correlation analysis, and regressive analysis were used to compare the differences between this group and a reference population of adults with normal occlusion. The results indicated that the maxillary base is shorted and in a more retrusive position in adults with Class III malocclusion. Furthermore, the mandible is in a more protrusive position. The pattern of growth in the Class III malocclusion was found to be horizontal, that is, hypodivergent. In diagnosis of the abnormality, the rule of the quadrilateral can provide assistance. That is, the maxillary base length, mandibular base length, anterior lower facial height, and posterior lower facial height can be used to evaluate the abnormality. In postural relationshops the sagittal angle, vertical sagittal ration, and horizontal sagittal ratio can be used as a reference.