A 65-year-old male admitted to the Anjo Kosei Hospital due to pancytopenia. The findings at the time of admission were; leukocyte count 2,000/microliters, erythrocyte count 1,580,000/microliters, and platelet count 88,000/microliters. Bone marrow specimen revealed mild hypocellularity with 26% of the blast cells. He was diagnosed RAEB in transformation. Chromosome analysis showed 46, XY, -7, +8, -17, + marker in three cells and 45, XY, -7, -17, + marker in two cells out of five cells. He was treated with the low-dose Ara-C (20mg/body s.c. injections every 12 hrs) for 9 days. Twenty five days later, the blast cells in the bone marrow decreased to 4%, and the complete remission was obtained. The duration of remission is 27+ weeks. At the time of complete remission, the bone marrow cells showed the normal karyotype. In this case, the effect of low-dose Ara-C to the blast cells may have not resulted from induction of differentiation but cytocydal action.