A 35-year-old woman attended our hospital with chronic myeloid leukemia and was prescribed imatinib mesylate. She was admitted with lower abdominal pain, stomatitis, and hyposthenia after an increase in her dose of imatinib mesylate. When the treatment was changed to interferon-alpha and Ara-C, the lower abdominal pain, stomatitis, and hyposthenia improved, but bone marrow aspiration showed 36.4% blasts. After the treatment was changed back to an increased dose of imatinib mesylate (800 mg), the stomatitis deteriorated and intestinal bleeding reoccurred. Endoscopy demonstrated the presence of multiple ulcers in the ascending colon and 99mTc RBC scintigraphy demonstrated lesions of the large and small intestine. The patient declined any treatment except for transfusion and died suddenly after ten days. The present case suggests that we should carefully consider the possibility of intestinal bleeding when prescribing imatinib mesylate.