Ears with chronic otitis media are characteristically associated with small mastoid air cell systems. This also indicates a small air volume in these ears, which may influence the pathophysiology and course. Therefore, it may be important to get a measure of the air volume in the middle ear mastoid air cell system. Conventional mastoid X-ray investigations have so far been used for estimating this volume. As impedance audiometry has gained in popularity during recent years, it has been postulated that this tool may be useful for measuring the volume in the middle ear and the mastoid system. In this report we present data comparing the acoustically measured size to direct volumetric and planimetric size of the mastoid air cell system on the X-ray film. Temporal bone specimens with perforation of the tympanic membrane have been investigated. A good correlation between these methods is obtained, and we conclude that impedance audiometry enables a rapid and valuable estimation of the air reservoir in the middle ear.