Twenty six children(44 ears, average age 6.4 years) who was diagnosed as middle ear effusion by impedance audiogram and otoscopic finding were included in this study during the period of November 1985 through August 1986. Impedance audiometry was performed shortly before general anesthesia and shortly before myringotomy. Follow-up(average duration 145 days) impedance audiogram was performed in 9 ears of negative myringotomy. The results were as follows : 1) Middle ear fluid was found in 34 ears(serous 17, mucoid 13, purulent 4), but not found in 10 ears. 2) Preanesthesia tympanogram in 15 ears(34 percent) was changed in premyringotomy tympanogram. 3) Static compliance in preanesthesia compared with premyringotomy decreased 0.01cc in the presence of middle ear fluid and 0.35cc in the absence of middle ear fluid. 4) Middle ear pressure in preanesthesia increased shortly before myringotomy, 70mmH₂O in the presence of middle ear fluid and 340mmH₂O in the absence of middle ear fluid. 5) Among 13 ears of which positive pressure greater than 100mmH₂O in premyringotomy tympanogram, middle ear fluid was found in 8 ears, but not 5 ears. 6) In 9 follow-up ears of negative myringotomy which were not inserted ventilation tube, 8 ears changed to normal tympanogram(type C to A in 6 ears, type B to A in 2 ears) and 1 ear not changed in type C.