Introduction: This study aimed to evaluate the possible effects of maxillary molar distalization on maxillary sinus mucosa thickness and sinus volume in 3-dimensions., Methods: The study was conducted with 46 patients: 73.9% (n = 34) females and 26.1% (n = 12) males. The patients had a mean age of 27.89 ± 6.62 years, ranging from 18 to 45 years. The study included patients who were admitted to the Faculty of Dentistry at Van Yüzüncü Yıl University between 2010 and 2021 and underwent cone-beam computed tomography (CBCT) before and after orthodontic treatment for various reasons. The CBCT scans of patients were divided into 2 groups: the first group underwent orthodontic treatment and maxillary molar distalization with Class II elastic, whereas the second group consisted of patients who did not get orthodontic treatment (control). In CBCTs, sinus mucosa thickness was measured on the anterior, posterior, medial, lateral, superior, and inferior walls of the maxillary right and left sinuses. Three-dimensional volume calculations were made using CBCT images taken before and after the treatment of both orthodontic treatment patients and control patients. Statistical significance was set at P <0.05., Results: The orthodontic treatment group had a mean distalization of 2.86 ± 0.27 mm. The mean distalization time was 8.01 ± 0.64 months. When the differences between the pretreatment and posttreatment CBCT scans were examined in terms of sinus mucosa thickness in the anterior, posterior, medial, lateral, and superior walls of the right and left sinuses, there was no statistically significant difference between treatment and control groups (P >0.05). The change in sinus mucosa thickness in the right and left sinus inferior wall from T1 to T2 was significantly higher in the control group than treatment group (P <0.05). In addition, the increase in maxillary sinus volume was statistically significantly higher in the treatment group than in the control group (P <0.05)., Conclusions: We observed that maxillary molar distalization caused an increase in maxillary sinus volume. The change in sinus mucosa thickness in the right and left sinus inferior wall was lower in the maxillary molar distalization group., (Copyright © 2023 American Association of Orthodontists. Published by Elsevier Inc. All rights reserved.)