To explore the relationship between the susceptibility to arsenic trioxide (As(2)O(3))-induced apoptosis of leukemia cells and the level of reactive oxygen species (ROS) of cells, flow cytometry and electron microscopy were applied to identify apoptosis, and dihydrorhodamine123 was used to display the ROS level of cells. As(2)O(3) alone or in combination with 2,3-dimethoxy-1,4-naphthoquinone (DMNQ, 2.5 &mgr;mol/L for NB4 cells, 10 &mgr;mol/L for U937 cells) were used to induce cell apoptosis. The results showed that NB4 cells possessed higher level of ROS than U937 cells. DMNQ raised ROS levels of NB4 and U937 cells, sensitized U937 cells to As(2)O(3)-induced apoptosis, and enhanced the efficacy of As(2)O(3)-induced apoptosis of NB4 cells. Catalase reversed the effect of DMNQ on NB4 and U937 cells. It was concluded that the susceptibility of leukemia cells to arsenic trioxide-induced apoptosis is determined by ROS level in the cells.