Objective: To study the effects of constantly slow intravenous arsenic trioxide (As2O3) infusion regimen on decreasing leukocytosis in vivo and in vitro. Methods: Three kinds of leukemia cells, NB4, K562, and acute promyelocytic leukemia (APL) cells, were cultured in the media with constant concentration and varying concentrations of As2O 3 respectively for 24 hours. Seventy-five patients were enrolled in two groups randomly. In trial group, 37 patients received continuously slow intravenous As2O3 infusion regimen for 24 hours with an infusion rate of 8 drips per minute and total infusion duration of about 18-21 hours daily. In control group, 38 patients received routine regimen with an infusion rate of 45-55 drips per minute and total infusion duration of about 2-3 hours daily for 24 hours. The daily As2O3 dosage was 0.16 mg/kg. The intracellular arsenic concentration was measured by atomic fluorescence assay. The apoptosis rate of cells, CD33- CD11b + cells, and CD33+ CD11b- cells were monitored by flow cytometry. Results: The apoptosis rates of NB4, K562, and APL leukemia cells in the media with constant As2O3 concentration were 56.6% ± 2.4%, 27.6% ± 3.1%, and 52.2% ± 2.8%, respectively, which were significantly higher than those with changing As2O 3 concentration (23.2% ± 2.1%, 11.0% ± 2.5%, and 21.0% ± 2.5%, respectively, P < 0.01). The apoptosis rates of APL, M2 type acute myeloid leukemia (AML-M2), and chronic myeloid leukemia (CML) patients in the trial group (28.5% ± 1.9%, 9.5% ± 0.6%, and 12.5% ± 1.8%) were also significantly higher than those in control group (8.5% ± 2.2%, 2.9% ± 0.8%, and 4.5% ± 1.2%; P < 0.05). The ratios of CD33+ CD11b- and CD33- CD11b+ cells in control group were significantly higher than those in trial group. Conclusion: The continuously slow intravenous As2O3 infusion regimen can obtain high efficiency of apoptosis and low differentiation proportion, relieve leukocytosis, and gain maximal therapeutic benefit.