Bronchiolitis obliterans organizing pneumonia (BOOP) is often diagnosed in patients with pneumonia who respond poorly to antibiotics. BOOP is often idiopathic, and the etiology of the remaining cases has been attributed to a wide range of agents or medical conditions. When a patient develops the clinical symptoms characteristic of BOOP, the medical team must endeavor to determine the etiology of this disease because it can be treated with glucocorticoid and avoidance of the causative agent. In particular, if BOOP is diagnosed during or after chemotherapy for a malignancy, the possible culprit agent can be the anti cancer drugs but other drugs used for supportive care must be also be considered. We report a case of BOOP that arose after CHOP chemotherapy and a filgrastim injection in a patient with a diffuse large B-cell lymphoma.