A 9 year old boy with acute lymphoblastic leukemia (ALL) received recombinant human granulocyte colony-stimulating factor (rhG-CSF) and showed a marked increment of myeloblasts in the peripheral blood. He was administered repeated courses of intermediate-dose cytosine arabinoside (Ara-C) therapy (1500 mg/m2, days 1-5) for frequent central nervous system (CNS) relapse of ALL. The peripheral white blood cell nadir was less than 1000/microL, so he was treated with rhG-CSF. A marked increment of peripheral blood blasts was noted 3-5 days after rhG-CSF treatment. These cells decreased with the appearance of mature myeloid cells and disappeared about 2 weeks after the start of treatment. These findings suggested that the blasts might have the ability to differentiate into mature myeloid cells. A control patient with repeated CNS relapse of ALL showed no increment of peripheral blood blasts after similar repeated courses of Ara-C without rhG-CSF treatment. Cultured peripheral blood blasts obtained from the present patient showed differentiation into mature myeloid cells by morphological studies and surface marker analysis. These findings indicate that the peripheral blood blasts drawn by G-CSF were not leukemic blasts but normal myeloblasts.