OBJECTIVE To investigate the expression of AML1/ETO9a isoform in the acute myeloid leukemia (AML)-M2 patients. METHODS Expressions of AML1/ETO fusion gene and AML1/ETO9a isoform were detected by using reverse transcriptase-polymerase chain reaction (RT-PCR) in leukemia patients, MDS patients, leukemia cell lines and healthy subjects. Karyotype was studied by R-banding technique. RESULT In 30 newly diagnosed AML-M2 patients 15 were found to express AML1/ETO9a isoform, while the rest including 20 AML-M2CR, 18 other subtypes of AML, 5 chronic myelogenous leukemia (CML), 3 myelodysplastic syndromes (MDS), 3 leukemia cell lines (NB4, KG-1, K562) and 5 healthy subjects were AML1/ETO9a negative. Among the 15 AML/ETO9a isoform expressing cases, 13 were demonstrated t(8;21) translocation and AML1/ETO expression. CONCLUSION Isoform AML1/ETO9a was correlated to AML/M2, and it may promote the development of leukemia in combination with the AML1/ETO fusion gene.