Objective <p>The aim of this study was to evaluate the effects of the Mandibular Advancement Appliance (MAA) system on mandibular and maxillary volumes in growing individuals using three-dimensional cone-beam computed tomography (CBCT), and to compare the results with those obtained using a conventional functional appliance (Activator) and a control group.</p> Methods <p>This single-center, randomized, controlled study included 55 individuals with skeletal Class II malocclusion at cervical vertebral maturation stages 2 or 3. The MAA and Activator groups each comprised 20 participants, while the control group included 15 participants. Volumetric analyses were performed using CBCT data and Mimics 15.0^® software. Statistical analyses were conducted using SPSS version 21.</p> Results <p>An increase in mandibular and maxillary volumes was observed in both the MAA and Activator groups during the treatment period. However, these increases were not statistically significant (<i>p</i> &gt; 0.05). No significant differences were found between the treatment groups and the control group.</p> Conclusions <p>Within the scope of this short-term treatment protocol, the MAA and Activator systems demonstrated similar effects on maxillary and mandibular volumes. Considering the esthetic and comfort-related advantages of the MAA, this system may be an effective alternative for the treatment of Class II malocclusion.</p> Trial registration <p>ClinicalTrials.gov NCT07056829 (registered June 29, 2025; retrospectively registered).</p>