Abstract Objectives: Although there have been some reports that measured the size of the mastoid pneumatization, only a few studies have reported the age-related variations in the mastoid air cell system using 3-dimensional (3-D) reconstruction techniques of computed tomography (CT) images. This study presents that 3-D reconstruction techniques of CT images can be used to measure the volume of mastoid pneumatization. In adddition, normal development of the mastoid air cell system in Koreans was evaluated. Methods: Retrospective study. A 3-D reconstruction based on the image data obtained from CT was performed in the 205 ears of 105 patients without otologic diseases in order to measure the volume of mastoid pneumatization by a surface-rendering algorithm on a personal computer. Results: The mastoid pneumatization continued to grow until the third decade. Thereafter, it declined slowly, and then rapidly after the seventh decade. The mean volume of the mastoid pneumatization was 7095.2 mm 3 in Korean adults. No statistically significant difference was found between male and female or between right and left sides. There was a significant difference between the larger and smaller sides of individuals. Conclusion: The volume measurement technique based on the 3-D reconstruction technique reported here is widely available, highly accurate, and easy to perform. The results of this study may be helpful in evaluating of the mastoid air cell system and for managing mastoid diseases.