We present the case of a 72-year-old man, who had begun taking oral salazosulfapyridine for the treatment of ulcerative colitis 14 years earlier. He developed a fever and cough and was diagnosed with bacterial pneumonia. Treatment with antibacterial agents was started at our department, but his condition did not improve. Salazosulfapyridine was discontinued and a bronchoscopy carried out, revealing a predominance of eosinophils in the cell fraction of the bronchoalveolar lavage fluid. Histopathologic findings from lung obtained by transbronchial lung biopsy showed reactive enlargement of alveolar epithelial cells and formation of Masson bodies within the alveolar lumen. Steroid therapy was then started, and the patient’s condition improved, but a pneumothorax developed on the right side. Surgery was performed and the steroid dose gradually reduced to zero. Pathologic examination of the transbronchial lung biopsy and surgical specimens revealed signs of organizing pneumonia. A drug-induced lymphocyte stimulation test for salazosulfapyridine was negative. When treatment with salazosulfapyridine was accidentally resumed at another medical facility, the lung lesion did not recur. A diagnosis of lung disease associated with ulcerative colitis was made. During the course of treatment, a bulla formed, leading to a pneumothorax due to air leakage from the area formed by the bulla. The presence of a scar in the right middle lobe and contraction of the right lower lobe due to the organizing pneumonia seem to have contributed to the bulla formation.