To study cell cycle kinetics of acute leukemia cells and to find out its relationship with clinical treatment outcomes, flow cyto-metric measurements of cell cycle kinetics, especially percentage of S-phase cells (S%) and duration of S-phase(Ts), were made in bone marrow cells of 39 patients with acute leukemia, including 26 cases of ANLL and 13 cases of ALL. Among them 31 cases were at diagnosis and 8 were in relapes. Bromodeoxyuridine(BrdU) / DNA biparameter analysis was used, in which cells were stained with propidium iodide(PI) to estimate total cellular UNA content and a monoclonal antibody against BrdU as a probe for BrdU incorporated into DNA. The results showed that the mean S% of bone marrow cells was significantly lower than that of normal controls, while there was no significant difference between S% of patients at diagnosis and of those in relapse. However, the mean Ts was not significantly different in patients and in normal subjects, but Ts of patients in relapse was significantly shorter than that of the patients at diagnosis and that of normal controls. It is suggested that the proliferation rate of leukemic cells is lower than normal. Shorter Ts indicates faster regrowth of leukemic cells, which is closely related to relapse.