Objective To investigate apoptosis in vivo in patients with leukemia at different stages of the first cycle of chemotherapy.Methods We detected apoptosis of HL-60 cells and peripheral blood leukemia cells in 17 patients at different stages, using in situ terminal deoxynucleotidyl transferase (TdT) fluorescence measurement and DNA electrophoresis. Results When HL-60 cells were incubated with 0.02 mg/L harringtonine for 0 to 48 hours, agarose gel electrophoresis showed that DNA ladder patterns became evident only at 12 hour into the treatment. In situ TdT assay showed that apoptotic cells occurred after one hour of the treatment. Apoptotic cells were few (0-3.3%) before chemotherapy, but increased substantially (11.4%-87.5%) during chemotherapy in patients with complete remission (CR) or partial remission (PR). Apoptotic cells were few (0-6.1%) during chemotherapy in ten patients with no remission (NR). DNA ladder cannot be detected by agarose gel electrophoresis either before, during or after chemotherapy. Wilcoxon signed rank test shows: P=0.0012<0.01, apoptotic cells during chemotherapy were present in greater quantity than prior to chemotherapy. Wilcoxon rank sum test shows: P=0.0011<0.01, with the median of apoptotic cells during chemotherapy in patients with CR or PR more than with NR. Conclusions TdT assay can be used to detect apoptotic cells earlier and more sensitively than DNA agarose gel electrophoresis. In situ TdT assay is useful to detect apoptosis in vivo in the initial phase of chemotherapy for immediate modification of the chemotherapy regimen, whereas electrophoretic analysis is not sensitive enough to detect apoptotic cell in vivo. Where the median of apoptotic cells during chemotherapy in patients with CR or PR were greater than with NR, only effective drug therapy could induce apoptosis. Objective To investigate apoptosis in vivo in patients with leukemia at different stages of the first cycle of chemotherapy. Methods We detected apoptosis of HL-60 cells and peripheral blood leukemia cells in 17 patients at different stages, using in situ terminal deoxynucleotidyl transferase (TdT) Results when HL-60 cells were incubated with 0.02 mg / L harringtonine for 0 to 48 hours, agarose gel electrophoresis showed that DNA ladder patterns became evident only at 12 hours into the treatment. In situ TdT assay showed that apoptotic cells occurred after one hour of the treatment. Apoptotic cells were few (0-3.3%) before chemotherapy, but increased substantially (11.4% -87.5%) during chemotherapy in patients with complete remission (CR) or partial remission (PR). Apoptotic cells were few (0-6.1%) during chemotherapy in ten patients with no remission (NR). DNA ladder can not be detected by agarose gel electrophoresis either before, duri Wilcoxon signed rank test shows: P = 0.0012 <0.01, apoptotic cells during chemotherapy were present in greater quantity than prior to chemotherapy. Wilcoxon rank sum test shows: P = 0.0011 <0.01, with the median of apoptotic cells during chemotherapy in patients with CR or PR more than with NR. Conclusions TdT assay can be used to detect apoptotic cells earlier and more sensitive than DNA agarose gel electrophoresis. In situ TdT assay is useful to detect apoptosis in vivo in the initial phase of chemotherapy for immediate modification of the chemotherapy regimen, and electrophoresis analysis is not sensitive enough to detect apoptotic cell in vivo. Where the median of apoptotic cells during chemotherapy in patients with CR or PR were greater than NR, only effective drug therapy could induce apoptosis.