The relative factors that influence the extent of mastoid pneumatization and the position of the lateral sinus, ie, the postinflammatory strengths of the(environmental) factor versus a genetic factor, are still controversial. During middle ear surgery, the position of lateral sinus affects the operative procedure and postoperative complications ate. So, the authors measured the shortest distance from the posterior border of the external auditory canal to the anterior border of the lateral sinus, as seen on radiographs taken in the lateral projection of 190 cases with normal mastoid pneumatization and 190 cases with sclerotic mastoid, retrieved from a patient population suffering from chronic otitis media, and quantified and looked for a correlation between the amount of mastoid pneumatization and the distance of the lateral sinus from the external ear canal. The results were as follows : 1) The mean distance of the lateral sinus from posterior wall of external ear canal was 14.62±2.20mm in pneumatic type, 12.11±2.32mm in sclerotic type, short in sclerotic type with statistical significance(p<0.01), and there were no differences between males and females in normal or sclerotic type. 2) In the adult group, above 18 years of age, beyond the final stage of pneumatization, the mean distance was 14.77±2.22mm in normal pneumatic type(150 cases), and 12.30±2.18mm in sclerotic type(150 cases), short in sclerotic type with statistical significance(p<0.01), and under 17 years old, the mean distances was 14.10±2.08mm in normal pneumatic type(40 cases), and 11.38±2.35mm in sclerotic type(40 cases), and short in sclerotic type with statistical significance(p<0.01). 3) The correlation between the amount of mastoid pneumatization and the distance of the lateral sinus from the external ear canal in adult group with normal pneumatization shows that the smaller the pneumatization, the shorter the distance from the external ear canal to the lateral sinus with statistical significance(r=0.36, p<0.01).