The relation between the onset of chronic middle ear inflmmation and the degree of pneumatization was investigated in porcine tympanic bullae, which closely resemble the human mastoid air cell system. Pneumatization was inhibited in all inflamed ears, and the later the induction of otitis media, the lesser the degree of inhibition of pneumatization. It was concluded that chronic middle ear inflammation inhibits the development of the middle ear air cell system, and the time of onset plays an important role in the degree of pneumatization.