Superoxide dismutase (SOD) constitutes an important enzymatic defense against the destructive biochemical events associated with inflammation. To examine the presence of SOD in the middle ear fluid (MEF) in secretory otitis media (SOM) the content of SOD was measured in the MEF from 66 children with SOM. The applied method was based on light emission associated with reduction of the chemiluminescence of luminol reacting with superoxide anions (O-). From the inhibition exerted by the addition of standard SOD the concentration of SOD in the samples was extrapolated. The findings divided the MEF's into two significantly different groups: 48% with undetectable SOD, 52% with a SOD range of 200-12,000 ng/ml, median: 3,333 ng/ml, 95% confidence limits: 2,000-5,000 ng/ml (p < 0.0005). It is suggested that the concentration of superoxide dismutase in the middle ear may play an essential role for the outcome of secretory otitis media and the effect of insertion of ventilation tubes.