A 36-year-old woman presented with a 2-month history of dry cough, bilateral pain in the upper chest, and low-grade fever. She had a 1-year history of ulcerative colitis (UC), which was treated with mesalazine. Cultures of sputum and bronchoalveolar lavage (BAL) fluid were negative. Chest radiograph and a computed tomography (CT) scan showed dense bilateral subpleural infiltrates in both upper lobes. A c-ANCA test was positive in a 1:1280 titer, and further specification showed antibodies against proteinase-3 antigen. Due to the possibility of mesalazine toxicity, this medication was stopped. Within 2 weeks, the patient's symptoms markedly improved, together with the chest roëntgenogram.