We report a chronic myelomonocytic leukemia (CMML) patient whose initial symptom was pleural effusion, which is extremely rare. A 61-year-old male was referred to our hospital because of leukocytosis with blasts and pleural effusion with chest pain. Bone marrow examination showed trilineage dysplasia with 14% blasts and a normal karyotype. He was diagnosed as having MDS (RAEB) and infectious pleuritis on admission. Despite administration of antibiotics, leukocytosis with monocytosis and pleural effusion progressed rapidly. His diagnosis was then changed to CMML-2 and pleural infiltration due to leukemic cells expressing CD13, CD14 and CD33. After the leukocytosis was brought under control with hydroxycarbamide, the patient's pleural effusion disappeared.