Objective: To detect the G-CSF effect and its mechanism in hyperleucocytosis of patients with APL after treatment with all-trans retinoic acid (ATRA).Methods: MTT method was used to measure the proliferation of HL-60 cells or primary APL cells.cDNA-mRNA dot blot hybridization was used to detect the c-myc gene expression level of HL-60 cells after ATRA treatment. Cycle shift of HL-60 cells was measured by flow cytometric Analysis. Results: ATRA could inhibit obviously in vitro the proliferation of HL60 cells of primary APL cells. The G0/G1 ratio of HL-60at 24 hours or 96 hours post-treatment with ATRA was increased, proliferation of HL-60 cells at the same time was promoted obviously under the condition of rhGCSF. C-myc gene expression was analyzed by dot blot hybridization; its expression levels of 8, 24 or 96 hours in culture with ATRA separately were higher, low, high or lower as compared with each other. Conclusion: GCSF plays an important role in hyperleucocytosis of patients with APL after treatment with ATRA.