In order to investigate whether apoptosis occurs in vivo in patients with leukemia in different stage (before, during and after chemotherapy). We detect apoptosis of peripheral blood leukemia cells in 10 patients before, during and after chemotherapy with in situ TdT fluorescence measurement and DNA electrophoresis. The results showed that apoptotic cells were few (0-2.9%) before chemotherapy, 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 four patients without remission. Therefore, only effective drug therapy could induce apoptosis. DNA ladder cannot be detected by agarose gel electrophoresis either before, during and after chemotherapy. So we considered that in situ TdT assay is possible to apoptosis in vivo in the early course of chemotherapy for immediate modification of chemotherapy, while electrophoretic analysis is not sensitive enough to detect apoptotic cells in vivo.