BACKGROUND AND OBJECTIVES: The purpose of this study was to evaluate the effects of middle ear lesions on pre and postoperative hearing in patients with chronic otitis media (COM) and to assess the relationship between various prognostic factors and pre and postoperative hearing using more than 2000 COM surgery records retrieved from a computerized database released by the Korean Otologic Society in 2005. SUBJECTS AND METHODS: Data of 2312 COM surgeries, which had been conducted by a single surgeon from 1989 to 2009, were reviewed retrospectively. Mean preoperative and postoperative air-bone gap (ABG) was analyzed using a four-frequency (500, 1000, 2000, and 3000 Hz) pure tone average (PTA). Pre and postoperative ABGs were compared with regard to several prognostic factors, including disease duration, age, Eustachian tube function, existence of otorrhea, size of tympanic membrane perforation, degree of ossicular destruction, presence of cholesteatoma, and its type. RESULTS: Mean pre and postoperative ABGs of PTA in the 2079 cases studied were 27.88+/-13.33 dB and 17.38+/-15.64 dB, respectively, with a hearing success rate of 69%. The results suggested that preoperative ABGs were affected by Eustachian tube function, Griffin grade, presence of cholesteatoma and its type, and the degree of ossicular destruction. Hearing gains differed significantly among the groups when compared with factors of Griffin grade, presence or type of cholesteatoma, and the degree of ossicular destruction. CONCLUSIONS: Many factors affected pre and postoperative ABGs and postoperative hearing outcomes. These results will be useful for designing future clinical studies on COM surgery, as well as for surgical planning and patient counseling in clinical practice.