Objective <p>This study aimed to determine if tonsillar hypertrophy (TH) influenced the postsurgical skeletal stability of Class III patients receiving two-jaw orthognathic surgery.</p> Materials and methods <p>Sixty patients with skeletal Class III malocclusion corrected by conventional two-jaw surgery were included in the study. Patients were divided into the TH group (<i>n</i> = 30) and normal tonsil (NT) group (<i>n</i> = 30) depending on tonsil size on the presurgical cone beam computed tomography. Serial lateral cephalograms were traced and analyzed at 3 stages: before surgery (T0), within 2 weeks of surgery (T1), and 1 year after surgery (T2). Linear and angular cephalometric parameters were used to evaluate the surgical change (T0-T1) and postsurgical relapse (T1-T2). The data were analyzed with the paired t-test, independent t-test, chi-square test, correlation analysis and regression analysis.</p> Results <p>From T0 to T1, the TH and NT groups showed a similar surgical pattern of the maxilla and mandible for repositioning. From T1 to T2, significant differences in skeletal relapse were observed in the mandible horizontally but not in the maxilla between the 2 groups. Compared to the NT group, the TH group exhibited significantly more forward movement at point B as well as a greater increase in SNB angle one year after surgery.</p> Conclusions <p>Skeletal Class III patients with TH presented less skeletal stability of the mandible following two-jaw orthognathic surgery compared to those with NT. The factor of TH needs to be considered when orthognathic surgery is indicated in Class III patients.</p>