We have performed microsurgery of 859 cases of patients with middle ear and mastoid diseases between March 1980 and February 1988. The charts of 644 patients with chronic otitis media who underwent microsurgery were reviewed. The following results were obtained. 1) The total cases of operation were 859. Of this group. 644 cases(75.0%) were chronic otitis media without cholesteatoma. 2) Sex ratio was 1.1：1, 336 cases(52.2%) were female and 308 cases(47.8%) were male. 3) Site incidence of operated ear : Right side were 335 cases(52.0%) and left side were 309 cases(48.0%). 4) The age range was from 9 years to 67 years, 560 cases(86.9%) were in the 20 to 40 years age group. 5) Character of otorrhea : Dry up status were 394 cases(61.2%), seromucionous status, 238 cases(36.8%), and purulent status, 12 cases(1.9%). 6) Site of drum perforation were 337 cases(52.3%) in cental perforation, and 195 cases(30.3%) in total perforation. 7) Conditions of middle ear mucosa, 421 cases(65.4%) in hyperemic edematous status, 207 cases(32.1%) in normal condition and 16 cases(2.5%) in graulation condition. 8) Postoperative hearing results were mostly unchanged hearing in 69.9%(450 cases), which showed gain or loss less than 10 dB of air conduction. 9) X-ray findings of temporal bone were 296 cases(46.0%) in diploic, 196 cases(30.4%) in sclerotic and 152 cases(23.6%) in pneumatic. 10) The most common condition of ossicles were 482 cases(74.8%) with intact ossicles, and the second was the case of only absence of incus, 65 cases(10.1%), and the third was the case of only presence of stapes, 52 cases(8.1%). 11) The methods of operation were as follows : Intact canal wall tympano-mastoidectomy were 433 cases(67.2%), Tympanoplasty s mastoidectomy, 104 cases(16.1%). Besides, canal wall down tympanomastoidectomy, radical mastoidectomy and modified radical mastoidectomy were done. 12) Postoperative complications were 47 cases(7.3%). Most of them were retroauricular wound infections.