An 83-year-old male was admitted to our hospital because of pancytopenia and low grade fever on April 19, 1993. On admission, hematological data were as follows: WBC 1,000/microliters with 19% neutrophils, RBC 367 x 10(4)/microliters, Hb 9.5 g/dl and platelets 6.7 x 10(4)/microliters. Bone marrow examination revealed 6.6% myeloblasts and 33.5% erythroblasts. Morphological abnormalities included hypersegmentation, degranulation and pseudo-Pelger's nuclear anomaly in neutrophils. Based on these findings the diagnosis of refractory anemia with excess of blasts (RAEB) of the myelodysplastic syndrome (MDS) was made and therapy with low dose Cytarabine (Ara-C) was initiated in April 1993. The patient had two episodes of severe pneumonia in June and July. Therefore, 75 micrograms/day of G-CSF was given in addition to antibiotic therapy for the second episode of infection in July. Thereafter the severe infection subsided, and G-CSF administration was switched to an intermittent schedule (75 micrograms twice a week) since September. Cytarabine ocfosfate (100 mg/day) was added for 10-14 days at interval 1-2 months from October,1993. He has been well with no episode of infection for more than two year. One major concern regarding the clinical application of G-CSF in MDS patients is related to the possible stimulation of leukemic cell proliferation. Frequent hematological monitoring is necessary in patients with RAEB who are prone to develop acute myeloid leukemia. However, we administered G-CSF at a relatively low dose twice a week for over two year and could successfully prevent infections without inducing the leukemic changes.