The patient was a 64-year-old woman who had undergone partial enterectomy for a small intestinal tumor in August 2005, and gastrointestinal stromal tumor (GIST) was diagnosed. Administration of imanitib mesylate was initiated as postoperative chemotherapy in November 2005. In February 2006, a slight ground-glass opacity was noted in the right lower lobe on chest CT. In April, cough and dyspnea appeared, and non-segmental reticular ground-glass opacity was noted in bilateral lung fields. Drug-induced pneumonia associated with imatinib mesylate was suspected based on the clinical course, and administration of imatinib mesylate was discontinued. Oral administration of 30mg prednisolone was initiated, and the symptoms and shadows on X-ray films improved. The steroid dose was gradually reduced, and recovery of the patient was smooth. Imatinib mesylate is anticipated to be a good potential therapeutic drug for interstitial pneumonia because it blocks PDGF receptors. However, the risk of imatinib-induced interstitial pneumonia should be noticed.