Mesalazine(5-Aminosalicylic acid,5-ASA), a component of Sulfasalazine (sulfapyridine bound to 5-ASA), is used to treat inflammatory bowel disease. Most adverse pulmonary effects caused by sulfasalazine have been attributed to sulfapyridine. However, lung toxicity associated with mesalazine(5-ASA) is rare. Here we report a case of eosinophilic pneumonia in a 44-year-old woman with ulcerative colitis, who was treated with mesalazine. She developed symptoms of a dry cough, mild night fever, and exertional dyspnea. Bilateral peripheral pulmonary infiltrates, peripheral blood eosinophilia, and histologic features were consistent with eosinophilic pneumonia. The symptoms improved quite rapidly after the discontinuation of mesalazine and initiation of steroid therapy. Therefore, adverse pulmonary reactions to mesalazine must be considered in a differential diagnosis of pulmonary involvement in patients with inflammatory bowel disease who are receiving with mesalazine therapy.