BACKGROUND AND AIM. The mastoid air cell system (MACS) plays a pivotal role in the physiology of the ear, and pathophysiology of middle ear inflammatory disease. A well-pneumatized mastoid is necessary for normal functioning of the middle ear. However, mastoid pneumatization is said to be impaired by inflammatory/infective middle ear pathologies. In our environment, CSOM is a prevalent disease which commonly starts in childhood, the period of development of mastoid pneumatization. This study aims to determine the pattern of mastoid pneumatization in normal individuals, compare it with pneumatization in ears with CSOM, and to determine any relationship between age at onset of CSOM and pneumatization. SUBJECTS AND METHODS The study was carried out between October 2005 and July 2006 at the ENT Clinic in Lagos university teaching hospital (LUTH). A questionnaire was used to obtain bio data, history of CSOM, and physical findings in the ears. Participants had x-ray mastoids in schuller’s lateral projection. RESULT/CONCLUSION 2 One hundred and ten subjects (220 ears) were enrolled for the study. However 100 subjects (200ears) were analysed. A study group comprising forty-eight subjects with unilateral CSOM, and 52 subjects with apparently normal ears bilaterally as controls. The mean area of mastoid pneumaization in the controls was 6.45 ± 2.4cm2 as against 3.00 ±2.0cm2 in ears with CSOM. Fifty- seven (54.8%) ears of the control group had well pneumatized mastoid compared with 2(4.2%) in ears with CSOM. Sclerotic mastoid was seen only in ears with CSOM, and this was directly related to the age of onset and duration of CSOM. The findings in this study suggest that mastoid pneumatization in our environment is generally smaller in size than reported in Caucasians, and the pneumatization is impaired by CSOM, and worse when CSOM starts in childhood. KEY WORDS: Pneumatization, Mastoid air cell system Chronic suppurative otitis media