Reduced mastoid pneumatization is common in children who have otitis media with effusion(OME), but is still a subject of controversy whether hypocellularity is the cause or result of OME. The purpose of this study was to investigate the changes of the mastoid pneumatization after medical and surgical treatment and to detect the prognostic significance of the mastoid air cell system in the childhood OME. 120 health children from the age of 3 to the age of 14 years were observed as a control and then the degrees of pneumatization were classified as a very small, small, moderate, and large mastoid. 36 children with OME were seperated to two groups. One was the chronic OME group who had a mild hearing impairment more than 2 months, and the other was recurrent OME group. All were treated with only a conservative treatment for 3-6 weeks and then waited till 3 months. If the effusion continued after treatment, a surgical intervention was performed. The results were as followings : 1) In chronic OME patients, the recovery rate was 47% at third months. 25% of the very small mastoid and 50% of the small mastoid at initial visit improved in pneumatization. Only 15% of 13 unchanged cases in pneumatization after final therapy were recovered, but 86% of 29 improved cases in pneumatization were recovered. 2) In recurrent OME patients, the recovery rate was 43% at third months but decreased to 30% at ninth months. There were little changes in pneumatization after theray. 6% of 16 unchanged cases in pneumatization after final therapy were recovered, but 75% of 4 improved cases in pneumatization were recovered. In conclusion, the unchanged mastoid mastoid pneumatization as the state of a very small or small mastoid pneumatization after medical and surgical treatment suggests poor prognosis and high risk of recurrence.