Although most studies have agreed that small mastoid air cell systems correlate with long-standing otitis media, the extent to which the environmental factors affect the development of MACS remains undetermined. We investigated the radiographic development of mastoid air cell systems in children with recurrent acute otitis media or otitis media with effusion who were treated with ventilation tubes early in life. Prospective follow-up. Of 305 consecutive patients aged less than 17 months with recurrent acute otitis media or otitis media with effusion who received primary tympanostomy in the Central Hospital of Central Finland, 281 (92.1%) were followed for 5 years. Mastoid radiographic films were obtained preoperatively and at the 5-year end-point examination. Mastoid air cell system size was planimetrically measured. Small baseline mastoid air cell system size was associated with young age at the time of first diagnosis of otitis media and small size of the child. Slow growth and small final size of mastoid air cell system related significantly with female gender and with slow overall growth of the child. Slow mastoid air cell system growth was observed in children who required several ventilation tubes during the follow-up period. Small final mastoid air cell system size correlated significantly with unfavorable otological outcome. The genetically determined development of mastoid air cell system is arrested in varying degree by environmental factors, particularly by an early onset of otitis media, a long-standing disease, and an unfavorable otological outcome. On the other hand, poor pneumatization seems to be a risk factor for chronic and recurrent infections and, ultimately, for a deficient otological outcome. However, in a vast majority of patients, this untoward development may be reversed by an early and, if needed, repeated tympanostomy.