A 68-year-old man with chronic myelomonocytic leukemia (CMML) was initially treated with hydroxyurea with subsequent stable disease. In the time course, he developed bilateral pleuritis accompanied by leukocytosis and spiking fever. Cytologic analysis of the pleural effusion revealed abundant mature granulocytes and monocytes. He was treated with intravenous or oral etoposide with consequent resolution of the pleuritis, indicating the pleural involvement of CMML. Three months later, he developed hepatomegaly and became febrile. One month thereafter, the CMML transformed to acute myeloid leukemia, and the patient developed massive bloody stools associated with epigastric pain and leukocytosis. A gastrofiberscopic examination showed multiple bleeding gastric ulcers. The bleeding ulcers were treated with the clipping procedure; however, the bloody stools continued. Although intravenous etoposide was effective for the leukocytosis and hepatomegaly, the treatment did not improve the bloody stools. The patient finally died of panperitonitis. The autopsy showed multiple ulcers of the transverse colon, some of which were perforated. Microscopically, the ulcerated areas were densely infiltrated with leukemic cells predominantly consisting of immature monocytes and granulocytes. This patient may be the first reported case of CMML complicated by colonic perforation due to leukemic infiltration.