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The efficacy of mepolizumab is well documented in severe eosinophilic asthma (SEA), although the stringent selection criteria adopted by SEA clinical trials limits the generalizability of results. Our study evaluated the effectiveness and safety of mepolizumab in patients with SEA in Spain. The primary efficacy endpoint was the change in the rate of clinically significant asthma exacerbations 12 months after starting mepolizumab compared to the baseline rate in the 12 months prior to treatment. Patients were stratified by baseline blood eosinophil counts. We conducted a multicentric observational cohort study of SEA patients treated with mepolizumab across 24 specialized hospital asthma units in Spain. Severe exacerbation rate, lung function, oral corticosteroid use (OCS) and asthma control test (ACT) were retrospectively collected and compared during the 12-month pre- and post-mepolizumab treatment. Adverse events were also investigated. A total of 318 patients with SEA were included (mean age: 56.6 years, 69.2% female). Exacerbation rates decreased by 77.5%, and 50.6% of patients did not suffer any exacerbations during the 12 months of treatment. The difference in forced expiratory volume in 1 s (FEV1) pre- and post-bronchodilator after starting mepolizumab was 0.21 (0.46) L (95% CI 0.14-0.27) (p < 0.001). Exacerbations and lung function significantly improved across all eosinophil subgroups. Among the 98 patients on OCS, 47.8% were able to discontinue this treatment and the mean daily dose was decreased by 59.9%. The baseline ACT score was 14.1, increasing by a mean (SD) of 6.7 points (1.9) at 12 months. Adverse events related to mepolizumab were uncommon. This real-world study of SEA patients confirms that mepolizumab is effective in reducing clinically meaningful exacerbations, improving lung function, and decreasing OCS dependence and mean OCS dose at 12 months, irrespective of baseline eosinophil counts.