Hypereosinophilic syndrome is a group of diseases defined by marked eosinophilia in blood or tissue and eosinophil-related clinical manifestations. Benralizumab is a monoclonal antibody against interleukin-5 receptor α, which is expressed on human eosinophils. It is currently only licensed as an add-on maintenance treatment in adult patients with severe eosinophilic asthma inadequately controlled despite high-dose inhaled corticosteroids plus long-acting β-agonists.