Correlation Between Mandible and External Ear in Patients with Treacher-Collins Syndrome Background: Patients with Treacher-Collins syndrome (TCS) are frequently affected by congenital ear deformities. The external ear in patients with TCS tends to have both abnormal morphology and reduced overall volume. Previous studies considered a correlation exists between TCS mandibular skeletal features and external ear volume. The purpose of this study was to assess the external ear volume in patients with TCS 3-dimensionally. Furthermore, this study evaluated the relationship between mandibular morphology, external ear profile, and external ear volume. Methods: A total of 36 nonoperated patients with TCS were compared to 39 age- and gender-matched normal controls. Morphologic variables of the mandible and the external ear were compared between TCS group and controls by 3-dimensional cephalometrics. The external ear volume and morphologic variables were analyzed with independent sample T-tests and Pearson correlation coefficient analyses (level of evidence: level III). Results: The external ear volume was reduced by approximately 50% in patients with TCS compared to controls (P < 0.001). External ear length and width were positively correlated with external ear volume (length: r = 0.809, P < 0.001 on left and r = 0.732, P < 0.001 on right; width: r = 0.518, P = 0.001 on left and r = 0.447, P < 0.010 on right). A negative correlation of right ear inclination angle and external ear volume was shown in patients with TCS (r = −0.396, P = 0.027). However, no correlation was shown for the mandibular anatomic variables. Conclusions: Three-dimensional analysis confirmed that external ear volume is significantly reduced in patients with TCS. The external ear dimensions and orientation correlated significantly with ear volume. There was no intrinsic association between the severity of mandibular deformity and external ear volume.