Three patients with chronic myeloid leukemia (CML) in blastic transformation were treated with G-CSF plus middle dose cytosine arabinoside (Ara-C). G-CSF was administered (150 mg, s.c. or 300 mg, d.i.v./day) 24 hr prior to Ara-C (2-3 g/body, 6 hour d.i.v. for 2-5 days) and continued until the peripheral neutrophil count rose above 1,000/microlitre. As a supplement, VP-16 (80 mg/m2, for 2 days) was administered as warranted to control the growth of blastic cells. All 3 patients survived for more than 12 months with a favorable performance status. Normal karyotypes were detected in 2 of the patients after chemotherapy. One of those patients in paticular demonstrated normal bone marrow findings with the almost complete disappearance of the Ph-positive clone. In vitro cultures of peroxidase-negative CML blastic cells revealed that G-CSF stimulated the induction of blastic cells into the cell cycle and that blastic cell apoptosis was more pronounced in cells cultured with G-CSF plus Ara-C than with G-CSF or Ara-C alone. G-CSF plus middle dose Ara-C therapy appears to be a strong candidate for the treatment of CML in blastic transformation with a poor prognosis.