Expression of AML1/ETO mRNA was observed in bone marrow cells from 49 untreated leukemic patients, and continuously detected during different periods after chemotherapy (12 cases) or bone marrow transplantation (8 cases). The results showed that AML1/ETO mRNA could be expressed in cells from AML-M(2), AML-M(4) and MDS-RAEB-T patients. The positive expression changed into negative at different duration in patients who achieved complete remission either by chemotherapy (9 cases), allogeneic bone marrow transplantation (5 cases) and autologous peripheral blood stem cell transplantation (1 case), and they were sustained in complete remission status. In chemotherapeutic group, patients whose AML1/ETO expression turning from negative (2 cases) or faint positive (1 case) to positive relapsed later. Two patients treated with Allo-BMT showed continuously positive results and died of GVHD and relapse, respectively. These observations suggest that AML1/ETO chimeric mRNA could disappeared after chemotherapy or bone marrow transplantation. The patients have a great probability to relapse if the results of RT-PCR are continuously positive or change from negative to positive. Regular detection is necessary for leukemic patients.