A 62-year-old man with a medical history that included artificial pneumothorax therapy at the age of 18 was admitted to our hospital because of persistent cough. Chest computed tomographic scans disclosed atelectasis in the right lung and pyothorax surrounded by calcifications. Radiographic examination failed to disclose any tumors. After admission, high grade fever developed due to aggravated pyothorax infection. Because antibiotic therapy and drainage failed, open window thoracostomy was performed. Tumors were found along the wall of the pyothorax cavity, and examination of resected specimens yielded a diagnosis of non-Hodgkin's lymphoma, diffuse large cell type (B-cell lineage). It was difficult to close the pyothorax cavity due to infection and lymphoma. Therefore, with the thoracic window open, the patient was given combination chemotherapy including CHOP (6 courses) and DeVIC (7 courses). He died of disseminated intravascular coagulation 17 months after thoracostomy. In patients with pyothorax associated lymphoma, chemotherapy is sometimes difficult to perform because of persistent pyothorax infection. Although edema and ascites due to protein loss from the tumor complicated the treatment of our patient, we concluded that open window thoracostomy is effective in managing pyothorax prior to and during chemotherapy.