Background: Mepolizumab 300 mg is an approved treatment option for patients with eosinophilic granulomatosis with polyangiitis (EGPA), yet, the adequacy of 100 mg of mepolizumab in disease control is controversial. Objective: To evaluate the sinonasal and respiratory outcomes of EGPA patients treated with 100 mg mepolizumab for one year. Methods: Evaluations of 11 patients were made of the sinonasal outcome test (SNOT-22) (nasal, otologic, sleep, and emotional domains), asthma control test (ACT), forced expiratory volume in 1 s (FEV1), blood eosinophil counts and oral steroid doses before mepolizumab treatment (T0) and at the 6(th) (T6) and 12(th) (T12) months. Results: A significant decrease was observed in the total SNOT-22 scores in the 6(th) month, after which the scores continued to be stable until the 12(th) month. (SNOT-22 median (IQR); T0: 70(53-82); T6: 19(4-35); T12: 11(6-40); T0-T6, p = 0.02; T6-T12, p = 0.85). In the subdomains of SNOT-22, nasal and sleep-related domains improved significantly in the first 6 months, and the otologic and emotional domains only improved from baseline in the 12(th) month. There was a significant decrease in blood eosinophil counts in the 6(th) month and oral steroid dose in the 12th month (eosinophils, median(IQR), T0: 1000(700-1800), T6: 100(0-200), p = 0.02; OCS dose, median(IQR), T0: 16(8-16); T6: 4(0-4); T12: 0(0-4); T0-T12, p = 0.002). A significant improvement was observed in ACT values in the 6(th) month (ACT median (IQR); T0:16(8-18); T6: 22(21-25); p = 0.01). Conclusion: Mepolizumab 100 mg provided a significant decrease in SNOT-22 values, especially in nasal and sleep domains, eosinophil counts and OCS dose in the 6(th) month.