We conducted aprospective, controlled study of patients with chronic otitis media and cholesteatoma (1) to examine the expression of myeloperoxidase (MPO) using immunohistochemical staining techniques and (2) to investigate the relationship between MPO activity and the degree of conductive hearing loss in these patients. Our study population included 51 adults-26 men and 25 women, aged 18 to 58 years (mean: 37.5)-who had been diagnosed with chronic otitis media and cholesteatoma by physical examination and computed tomography (study group). Another 30 patients-13 men and 17 women, aged 18 to 52 years (mean: 32.7)-who had chronic otitis media without cholesteatoma served as the control group. Following audiometric evaluations, all patients underwent appropriate surgery. Postoperatively, cholesteatoma samples were analyzed by immunostaining for MPO positivity as a marker for acute inflammation. We found that MPO activity was present in all 51 study patients (100%) but in only 10 controls (33.3%); the difference was statistically significant (p < 0.01). In the study group, the degree of MPO activity was slight in 6 patients (11.8%), moderate in 24 patients (47.1%), and intense in 21 patients (41.2%), while in the control group, all 10 MPO-positive cases showed only a slight degree of activity. We also found a statistically significant association in the study group between the degree of MPO activity and the degree of conductive hearing loss (chi(2) = 13.518; p < 0.001). We encourage further study of all steps in the process of cholesteatoma formation.