Purpose: Background: Eosinophilic lung disease is a known pulmonary complication of mesalamine; however, optimal management of affected patients is yet to be defined. Compared to anti-metabolite or biological agents, mesalamine compounds are easy to administer, relatively free of side effects, and are preferred for mild to moderate ulcerative colitis (UC). This is a case of a young patient with ulcerative colitis who developed eosinophilic pneumonia due to basalazine. After treatment with steroids and withdrawal of basalazine, the patient has been successfully maintained on sulfasalazine without reoccurrence of pulmonary symptoms or a flare of ulcerative colitis. Case: 23yo male with history of pan-UC presented to his primary care physician with sharp stabbing pain in right thorax exacerbated by taking deep breaths. He had no history of allergic or respiratory diseases, but was a half-pack per day smoker. His UC was in clinical remission on balsalazide since induction 6 years earlier. Patient had no history of treatment with steroids, anti-metabolites, or biological agents. Initial work up included a CT scan of the chest which revealed a right pleural effusion. Serologies and UA were unremarkable, except for anti-PR3-ANCA which was positive. Thoracentesis showed > 25% of eosinophils in fluid effusion. After intensive unrevealing testing, basalazine was withdrawn and patient was placed on prednisone with prompt resolution of his symptoms and pleural effusion. Repeat PR3-ANCA was negative. Steroids were tapered, and patient was placed on sulfasalazine without recurrence of pulmonary symptoms. His UC remained in remission throughout the allergic episode, and he has been successfully maintained on sulfasalazine for past two years without a flare. Conclusion: This case may represent the first published case of basalazineinduced eosinophilic pneumonia with successful reintroduction of sulfasalazine. In this case, basalazine may have induced an autoimmune phenotype manifesting as eosinophilic pneumonia, which was reversed by discontinuing the drug and subsequent disappearance of PR3-ANCA. It is unclear if patient's smoking status contributed to manifestation of eosinophilic pneumonia; however, smoking was not discontinued during this allergic episode. In conclusion, in rare cases of patients with UC who develop eosinophilic pneumonia induced by basalazine, a switch to another formulation of ASA after steroid treatment may be an option; however, further studies are required.