According to an editorial, after years of research into eosinophilic diseases and failure of treatment with anti–interleukin-5 antibodies in all patients with severe asthma, drug developers are increasingly using biomarkers to select patients who are most likely to have a response to a given treatment. It suggests that following this proof-of-concept study, additional research is needed to identify biomarkers that inform success and failure of mepolizumab in patients with eosinophilic granulomatosis with polyangiitis. It also calls for future trials to establish the appropriate dosing of mepolizumab, include participants with life-threatening eosinophilic granulomatosis with polyangiitis who were not included in this trial, and possibly evaluate synergy with immunosuppressants such as azathioprine and cyclophosphamide.