Background and ObjectivesZZIt is essential to understand gas physiology of mastoid cavity to study the pathophysiology of middle ear diseases, and the surface area and volume of mastoid mucosa are important parameters for evaluating gas physiology. However, the surface area and mastoid volume of the mastoid cavity have not been practically measured yet. Therefore, we measured and compared surface area and volume of the mastoid cavity before and after mastoidectomy using a virtual mastoidectomy model. Subjects and MethodZZWe performed a virtual mastoidectomy using 10 cases of temporal bone CT indicating pneumatic mastoid. First, we removed all air cells after outlining with irregular AOI function after loading axial CT images to Image-Pro Plus 4.0. Then we filled the removed area with equal planes using local equalization filter. Finally, we calculated and compared the total surface area, volume and area to volume (A/V) ratio by estimating their circumference and area. ResultsZZThe mean surface area of pneumatized mastoid cavity was 127.8 cm 2 (range: 94.2165.3 cm 2 ), and the mean volume was 7.1 cm 3 (range: 5.2-11.0 cm 3 ). The mean surface area and volume were altered to 42.8 cm 2 (range: 35.9-55.0 cm 2 ) and 12.6 cm 3 (range: 10.3-18.7 cm 3 ), respectively, after virtual mastoidectomy. As a result, the A/V ratio decreased from 18 to