We investigated the relationship between the Eustachian tube's ventilatory function and the pneumatization of mastoid air cells. In 65 adults (66 ears) with chronic otitis media (COM), ventilatory Eustachian tubal function was evaluated by the inflation-deflation test, while the area of mastoid air cells was measured by the rectangular method on X-ray film. Group I represents good function in both the inflation and deflation tests. Group II represents good function in the inflation test but poor function in the deflation test. Group III represents a poorly functioning Eustachian tube in both tests. The opening pressure in Group III was statistically higher than in Group I and Group II. The area of mastoid air cells in Group I was statistically larger than in Group III. There was no correlation between opening pressure and area of mastoid air cells in COM. Postoperative failure of the tympanic membrane was frequently found in Group III. We discuss the tubal function and pneumatization of mastoid air cells.