OBJECTIVE To study the mechanism of IFN on CML.METHODS Samples of 15 CML patients and 10 healthy controls were studied. The flow cytometry was performed to identify circulating pDCs. The concentration of IFN-α in serum and that in the supematant of peripheral blood mononuclear cells (PBMCs)cultured after stimulation with CpG ODN2216 were examined both in CML patients and in the healthy controls RESULTS There was significant reduction in the number of circulating pDCs, serum concentration of IFN-α and the capacity of IFN-α producing PBMCs in CML patients compared with those in healthy control individuals (P < 0.001). After the active treatment with IFN-α and hydroxyurea, the quantity and function of pDCs were increased in stabilized patients, especially the function of pDCs in 2 patients achieving major cytogenetic.response (MCR). The proportion and function of pDCs and the serum levels of IFN were inversely correlated with both WBC and age of the patients with CML, and positively correlated with the state of the illness.dysfunction of circulating pDCs. The active treatment with IFN in CML patients may be related to the restoration of pDCs.