Leukocytosis, splenomegaly, and an increased vitamin B(12) level are characteristic findings of chronic myelogenous leukemia in the chronic phase (CML-CP). Here, we report a patient with CML-CP accompanied by megaloblastic anemia. A 61-year-old man consulted our hospital because of anemia and thrombocytopenia. On physical examination, there were no remarkable findings; there was no hepatosplenomegaly. Laboratory findings were: hemoglobin 6.0 g/dl; MCV 113.6 fl; platelet count 100×10(9)/l; white cell count 8.66×10(9)/l; and LDH 1,236 IU/l. Peripheral blood smear demonstrated hypersegmented neutrophils and megalocytes with emergence of myeloblasts, giant metamyelocytes, and nucleated red cells. Vitamin B(12) and folic acid levels were low. Bone marrow examination showed megaloblastic change in the erythroblasts and myeloid hyperplasia. Following vitamin B(12) and folic acid administration, anemia and thrombocytopenia rapidly improved; thereafter, marked leukocytosis became evident. Based on the presence of t(9;22)(q34;q11) on cytogenetic study and a positive result for Major bcr/abl fusion gene, a diagnosis of CML-CP was established. This case illustrates that ineffective erythropoiesis results in anemia and thrombocytopenia in CML with vitamin B12 and/or folic acid deficiency.