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The objective of this study was to compare the prevalence of Bell's palsy in participants who underwent mastoidectomy (to treat chronic otitis media) and nonmastoidectomy participants (control).Using the national cohort study from the Korean Health Insurance Review and Assessment Service, mastoidectomy patients (2,045) and control participants (8,180) were matched 1:4 for age, sex, income, and region of residence. The prevalence of Bell's palsy in both the groups was measured from 0 to 10 years postoperation.In a sample of 1,025,340 Korean individuals, 7,070 were diagnosed or treated with Bell's palsy between 2002 and 2013; the annual incidence of Bell's palsy was 0.057%. The overall prevalence of Bell's palsy was three times higher in the mastoidectomy group (1.27%) than control group (0.49%) (p < 0.001). The prevalence of Bell's palsy was different between the two groups in postoperative 0 year: 0.78% for the mastoidectomy group versus 0.01% for the control group (p < 0.001).Although we could not verify the laterality, the prevalence of Bell's palsy was increased in chronic otitis media patients treated with mastoidectomy patients compared with controls, especially within a year after surgery.