AbstractA 50‐year‐old Japanese woman with advanced breast cancer presented with productive cough and dyspnea while she was receiving a sixth cycle of chemotherapy including atezolizumab. Chest computed tomography revealed bronchiolitis and transbronchial lung cryobiopsy revealed eosinophilic bronchiolitis. Corticosteroid therapy successfully resolved her symptoms. Eosinophilic bronchiolitis is a rare but important immune‐related adverse event; herein, we discuss its diagnosis and possible pathophysiology.