The fact that an association of the complement system in the pathogenesis of otitis media with effusion has been well documented after discovery of action mechanisms of the complement system in middle ear effusion. However, the exact mechanism of action, whether it acts as a defense mechanism against offending agents or triggers the inflammatory process by activated complement, has yet to be determined. To understand the role of the complement system in the pathogenesis of otitis media with effusion, the authors classified the otitis media with effusion according to its nature, serous(12 cases) or mucoid(26 cases) ; frequency of infection, recurrent(more than 2 times per year) or non-recurrent(less than 2 times per year) ; absence or presence of fever and otalgia, noninfectious or infectious. We analysed and compared the C3, C4 and total protein in each classification. The results were as follows ; 1) The C3, C4 and total protein level in serous otitis media were 30.4±10.9μg, 8.5±3.8μg and 1.90±1.10mg, respectively. 2) The C3, C4 and total protein level in mucoid otitis media were 47.1±21.1μg, 10.9±5.4μg, and 1.92±0.91mg, respectively. 3) In mucoid effusion the C3 level was higher than in serous effusion(47.1μg versus 30.4μg, p<0.05). 4) In non-recurrent otitis media with effusion the total protein level was higher than in recurrent otitis media effusion(2.3mg versus 1.3mg, p<0.01). 5) There were no significant statistical differences between middle ear effusion of non-infectious versus infectious patients.