Objectives: The goals of this study were to evaluate mastoid pneumatization using a grading system and to assess the prevalences of pneumatization of temporal structures of the temporomandibular joint (TMJ) and their relationships with age and sex using cone-beam computed tomography. Methods: For this study, 250 randomly selected patient dental volumetric tomography scans contained within the archives of the Department of Dentomaxillofacial Radiology, Gazi University Faculty of Dentistry were retrospectively reviewed, and the degrees of mastoid pneumatization were separately graded for the right and left areas. A grading system was used for pneumatization as follows: grade 0, pneumatization limited to the mastoid process; grade 1, pneumatization between the mastoid process and the glenoid fossa; grade 2, pneumatization between the deepest point of the glenoid fossa and the tip of the articular eminence; grade 3, pneumatization extending beyond the crest of the articular eminence. Results: Of the 500 investigated areas, 32.4 % had grade 0 pneumatization, 55.4 % had grade 1, 8.6 % had grade 2, and 3.6 % had grade 3. There were no significant correlations between pneumatization grade and age or sex. Conclusions Pneumatization of the mastoid process may extend beyond the external earway and include temporal components of the TMJ in a significant group of patients. © Japanese Society for Oral and Maxillofacial Radiology and Springer Japan 2013.