In order to investigate the suppressor of cytokine signaling-1 (SOCS-1) expression in peripheral blood mononuclear cells (PBMNC) of patients with acute and chronic myeloid leukemia and analyze its clinical significance, RT-PCR method was used for detecting SOCS-1 mRNA expression in PBMNC of 50 newly diagnosed patients. The result showed that positive expressions of SOCS-1 were observed in 4 of 25 patients with AML (16.00%), in 11 of 25 patients with CML (44.00%) and none in 10 normal controls. The differences between patients with AML and normal controls, and between patients with CML and normal controls were statistically significant. In CML group, 2 out of 12 cases with non-progression (chronic phase), 9 of 13 cases with progression showed the positive expression, the difference between two subgroups was statistically significant. Those CML patients with SOCS-1 mRNA expression had poor response to IFN-alpha. When they transformed into accelerated phase, SOCS-1 mRNA expression was more persistently and frequently observed, and no response to IFN-alpha was observed. Most of them had very poor prognosis. It is concluded that the SOCS-1 mRNA can be detected in the PBMNC of the patients with acute and chronic myeloid leukemia. The SOCS-1 mRNA expression in the patients with CML is higher than that in patients with AML, and it is higher in accelerated phase and blast crisis significantly. This phenomenon is highly related to the reaction of IFN-alpha and prognosis.