BACKGROUND: Mastoid pneumatisation is subject to numerous influencing factors including race, sex, and surrounding structures of the middle ear. This study aims to determine the mastoid air cell system (MACS) volume and its relationship with middle ear structures, and the influence of sex. MATERIALS AND METHODS: A cross-sectional study was performed analysing computed tomography (CT) scans in which MACS volume and the Eustachian tube length (ETL) were visible. MACS volume, ETL, and width and height of the aditus ad antrum were obtained. RESULTS: A total of 100 CT were included with a mean age of 38.5 ± 15.3 years, of which 56 were women and 44 were men. The mean right and left MACS volume were 5.43 ± 3.15 cm³ and 5.54 ± 3.43 cm³, respectively, with an ETL of 24.55 ± 3.07 mm in the right side and 24.24 ± 2.60 mm on the left side, and an aditus ad antrum width of 2.98 ± 0.65 on the right and 2.98 ± 0.58 on the left, and height of 4.51 ± 1.05 and 4.32 ± 0.85, on right and left side, respectively. There were statistical differences between sexes in left ETL, and in MACS volume bilaterally. A low positive correlation between aditus ad antrum height and MACS volume was identified. CONCLUSIONS: Mastoid pneumatisation was greater in men than in women. There was a low positive correlation between mastoid volume and ETL on both sides, and a significant correlation between right mastoid volume and aditus ad antrum height. This could lead us to believe that the length of ETL does not affect the pneumatisation of MACS.