PURPOSE: If significantly influenced by chronic otomastoiditis(COM), mastoid pneumatization and the position of the sigmoid sinus affect the operative procedure and postoperative complications in middle ear surgery. We evaluated mastoid pneumatization and sigmoid sinus position, and their relationship in COM, especially its during onset. MATERIALS AND METHODS: Using temporal bone CT and referring to any relevant medical records, we retrospect+tively analyzed 107 cases of COM and 49 cases of normal ear. The total case load comprised an adult group, aged above 16 years [100 cases of COM (M:F=46:54, mean age = 45 years), and 42 cases of normal ear,(M:F=20:22, mean age = 44 years)], and a childhood group, aged less than 16 years [7 cases of COM (M:F=4:3, mean age = 8.4 years), and 7 cases of normal ear (M:F=4:3, mean age = 7 years)]. We determined the thickness of the mastoid bone by measuring the shortest distance between the outer cortex of this bone and the deepest border of the sigmoid sinus; the depth of the sigmoid sinus; and the degree of mastoid pneumatization and sclerosis. Fifty-three patients whose medical history clearly included the onset of otomas-toiditis were divided into a child-onset group and an adult-onset group, and the relationship between the onset of otomastoiditis and the thickness of the mastoid bone was compared between the two groups. RESULTS: The mean axial thickness of the mastoid bone was 9.672 <+/-2.745 mm in COM and 12.430 +/-3.027 mm in normal ear. The difference was statisfically significant (p < 0.0001). The mean depth of the sigmoid sinus was 7.557 +/-1.868 mm in COM and 7.591 +/-2.315 mm in normal ear, with no statistically significant difference. In the childhood group, the mean axial thickness of the mastoid bone was 8.672 +/-2.978 mm in COM and 11.778 +/-3.087 mm in normal ear. This difference was statistically significant (p < 0.05). In the adult group, the corresponding figures were 9.742 +/-2.731 mm in COM and 12.538 +/-3.041 mm in normal ear, a dif-ference which was also statistically significant (p < 0.0001). Among patients with an obvious history of COM, child-onset cases totalled 24 (mean axial thickness of the mastoid bone, 9.2.0 +/-2.158 mm), while there were 29 adult-onset cases (mean axial thickness, 10.08 +/-2.99 mm). This difference in thickness between child-onset and adult-onset COM was statistically significant (p < 0.05). CONCLUSION: In COM, the degree of mastoid pneumatization is proportional to the axial thickness of the mastoid bone, and inversely proportional to the degree of sclerosis, anterior location of the sigmoid sinus and the onset of COM. If the sigmoid sinus is properly located, COM may inhibit mastoid pneumatization.