Among the several different asthma phenotypes, eosinophilic inflammation occurs in more than 50% of patients with either atopic or nonatopic asthma. High eosinophil counts, in both peripheral blood and the airways, are associated with recurrent disease exacerbations and severe airflow limitation [6]. Adultonset eosinophilic asthma is increasingly recognized as one of the most severe asthma phenotypes [7-10]. Another characteristic feature of adult-onset eosinophilic asthma is comorbid chronic rhinosinusitis with nasal polyps (CRSwNP), a feature known for many years and, in some cases, linked with aspirin and other nonsteroidal antiinflammatory drug hypersensitivities [11-13]. Concern regarding nonallergic, severe eosinophilic asthma and its associated comorbidities has increased Background/aim: Oral corticosteroid (OCS)-dependent severe eosinophilic asthma with chronic rhinosinusitis with nasal polyps (SEA-CRSwNP) would be a suitable phenotype for mepolizumab treatment. This study evaluated the short-term efficacy of mepolizumab treatment in OCS-dependent SEA-CRSwNP. Materials and methods: Baseline and 24th week results [daily OCS doses, asthma exacerbation frequency, asthma control test (ACT) scores, blood eosinophil levels, FEV 1 values, and numerical analog scale (NAS) of CRSwNP symptoms] of patients who were treated for at least 24 weeks with mepolizumab were retrospectively evaluated and compared. Results: A total of 16 patients were enrolled in the study. Mepolizumab was discontinued in one patient due to side effects. The daily OCS dosage was reduced from baseline in all patients, and at week 24 OCS was discontinued in 40% of the patients (baseline mean steroid dose: 9.2 ± 5.2 mg, 24th week: 1.3 ± 1.4 mg; P < 0.001). The number of asthma exacerbations within 24 weeks significantly decreased after beginning mepolizumab treatment (2.1 ± 2.7 vs. 0.07 ± 0.26; P = 0.012), and a significant increase in ACT scores (baseline mean ACT: 18 ± 5.7; 24th week mean ACT: 23.3 ± 3; P = 0.006) was observed despite the decrease in daily OCS dosages. There was no significant difference in FEV 1 values between baseline and week 24. Evaluation of the general symptoms of CRSwNP, as per NAS, revealed that the baseline mean NAS was 5.6 ± 4.4, and the 24th week mean NAS was 3.2 ± 3.2 (P = 0.021). Conclusion: This is the first real-life study evaluating the short-term efficacy of mepolizumab treatment on OCS-dependent SEA-CRSwNP. This study demonstrates that mepolizumab is an effective and safe biologic for the treatment of this severe asthma subphenotype.