Imatinib-mesylate (Gleevec, Glivec) is a protein-tyrosine kinase inhibitor that inhibits the Bcr-Abl tyrosine kinase created by the Philadelphia chromosome abnormality in CML. Imatinib is also used to treat patients with c-kit (CD 117)-positive unresectable tumors, or metastatic malignant gastrointestinal stromal tumors, or both. Imatinib is a well-tolerated drug with few side effects. However, it has been associated with gastrointestinal irritation, fluid retention and edema, skin rashes, depigmentation, hepatotoxicity, hemorrhage, and hematological toxicity (anemia, neutropenia, and thrombocytopenia). In addition, imatinib has been associated with dyspnea and cough, which are mainly secondary to the pleural effusion and pulmonary edema, which represent local or general fluid retention. These events appear to be dose related and are more common encountered in the elderly. However, there has been no report of hypersensitivity pneumonitis associated with imatinib-mesylate in Korea. We report a case of 51-year old woman who developed hypersensitivity pneumonitis that might have been induced by imatinib-mesylate during the treatment of a gastrointestinal stromal tumor.