OBJECTIVE To investigate the association between sinus tympani volume and petrous apex pneumatization in this retrospective-archival temporal bone computed tomography study. METHODS We included 46 temporal bones with pneumatized petrous apex from 26 patients and 52 temporal bones without petrous apex pneumatization from 26 other patients. Using OsiriX software for MacOS (version 3.8.1, Pixmeo), we measured the volumes from three-dimensional sinus tympani models and compared the sinus tympani volumes and depths between the temporal bones with and without pneumatized petrous apex. RESULTS Among 150 patients totally reviewed, 26 (17.3%) had petrous apex pneumatization. The median sinus tympani volume was 16.05 (5.6-59.7) mm3 in temporal bones with pneumatized petrous apex and 8.7 (1.76-59.7) mm3in temporal bones without. The sinus tympani volume was significantly greater in temporal bones with pneumatized petrous apex compared to those without (p < 0.001). Additionally, temporal bones with pneumatized petrous apex had a significantly deeper sinus tympani [median depth = 2.17 (0-3.04) mm] compared to the temporal bones without [median depth = 1.69 (0-3.73) mm] (p = 0.045). We found that petrous apex pneumatization was associated with deeper and larger sinus tympani. CONCLUSIONS Patients with pneumatized petrous apex had a greater sinus tympani volume associated with the increased risk of residual cholesteatoma.