Objective: The purpose of this study was to determine if free oxygen radicals (FORs) and antioxidant enzyme activities have some rote in pathogenesis of otitis media with effusion (OME) in children with adenoid hyperplasia. Methods: Seventy-four patients were enrolled in three groups of this study. The study group (Group 1) included 26 patients who had adenoidectomy with ventilation tube placement due to chronic OME. The control adenoid group (Group 11) consisted of 28 age-matched patients who had adenoidectomy without ventilation tube insertion. Twenty children were included in the healthy control group (Group 111). Erythrocyte matondialdehyde (MDA) levels, superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px) enzyme activities were investigated in the venous blood sample. Results: Erythrocyte MDA level and GSH-Px enzyme activity in the blood samples of study group (Group 1) were significantly higher than those of Group 11 and Group III (P < 0.05). SOD enzyme activity in the blood samples of Group I was significantly tower than Group 11 (P < 0.05), and were significantly higher than Group III (P < 0.05). CAT enzyme activity of Group I was significantly tower than that of Group III (P < 0.05). However, there was no statistically significant difference between Group I and Group 11 regarding CAT antioxidant enzyme activity (P < 0.05). Conclusions: The inflammation of the middle ear increases the level of FORs in erythrocyte. FOR level is normally maintained at a steady state by antioxidant enzymes. When the antioxidant defense system is weakened, the increased FORs may contribute to OME formation. We supposed that, antioxidant vitamins C and E, and scavenger enzymes such as CAT, SOD and GSH-Px may be added in the management of OME. (C) 2004 Elsevier Ireland Ltd. All rights reserved.