High-dose methylprednisolone (HDMP) treatment has been shown to induce the differentiation of myeloid leukemic cells to mature granulocytes in patients with acute promyelocytic leukemia and other subtypes of acute myeloblastic leukemia (AML). HDMP administration has also been shown to accelerate the recovery of leukocytes and increase bone marrow hematopoietic CD34+ progenitor cells in patients with acute lymphoblastic leukemia (ALL). In the present study, we evaluated the effect of short-course (4-day) HDMP treatment on peripheral blood (PB) CD34+ cells in patients with acute leukemia (AL). Fourteen children with AL who were receiving maintenance therapy were enrolled in this study. Methyl-prednisolone was given orally as a single daily dose of 30 mg/kg for only 4 days to nine children (three AML, six ALL) in whom chemotherapy-induced leukopenia (< 2 x 10(9)/L) was observed. Circulating CD34+ progenitor cells were determined by flow cytometry before (day 0) and 4 and 7 days after HDMP treatment. On days 4 and 7 after initiation of HDMP treatment, the number of PB CD34+ cells increased significantly (p < 0.05). Hematopoietic CD34+ progenitor cells were also determined in five patients with AL (two AML, three ALL) and chemotherapy-induced leukopenia who did not receive HDMP. The number of CD34+ cells was found to be significantly (p < 0.05) higher in the patients who received HDMP than in those who did not. Along with PB CD34+ progenitor cells, white blood cells (WBC), polymorphonuclear cells (PMN), and monocytes also increased significantly (p < 0.05) in patients treated with short-course HDMP. In conclusion, short-course HDMP treatment can be used to shorten the chemotherapy-induced leukopenic period in patients with AL, possibly by increasing the number of hematopoietic progenitor cells. Further studies are needed to evaluate the effect of this treatment on leukopenic patients with other malignancies.