The concentration of superoxide dismutase (SOD) in middle ear effusion (MEE) was measured by the electron spin resonance trapping method in order to clarify the role of SOD in otitis media with effusion (OME) in children. The SOD levels in mucoid MEEs were significantly higher than those in serous and purulent MEEs. There was no significant difference in the levels of SOD between neutrophil-dominant MEEs and mononuclear cell-dominant MEEs, and the levels were negatively correlated with the number of neutrophils in the MEEs. Moreover, the levels were significantly increased in patients having recurrence of MEE within 3 months compared with patients without recurrence of MEE. Although it is known that SOD plays an important role in protecting the host from oxygen radicals, the findings in this study suggest that SOD might be related to the chronicity of OME.