Objective To investigate whether apoptosis occurs in vivo in patients with leukemia before, during and after chemotherapy. Methods Apoptosis of peripheral blood leukemia cells in 10 patients in different periods was detected with in situ TdT fluorescence measurement and DNA electrophoresis. Results Apoptotic cells were few （0-2.9％） before chemotherapy, and increased obviously （11.4％-72.0％） during chemotherapy in patients with complete remission （CR） or partial remission （PR）, but decreased （0-2.9％） after chemotherapy （10-14 days）. Apoptotic cells were few （0-5.7％） during chemotherapy in 4 patients without remission. Conclusion Only effective drug therapy can induce apoptosis. DNA ladder cannot be detected by agarose gel electrophoresis before, during, and after chemotherapy. In situ TdT assay is useful to detect apoptosis in vivo in the initial phase of chemotherapy for modifying chemotherapy regimen, whereas electrophoretic analysis is not sensitive enough to detect apoptotic cells in vivo）.