To study the effect of curcumin on signaling pathway of signal transducers and activators of transcription (STAT5) in primary newly-diagnosed chronic myelocytic leukemia (CML) cells, and to explore the clinical significance of curcumin in the treatment of primary CML cells, the cells were randomly divided into 3 groups: normal control group, CML cells group, and curcumin group; the cellular proliferation was assayed by MTT test; the expression of cellular STAT5 mRNA in CML cells was detected by RT-PCR; the activation of STAT5 in CML cell was detected by electrophoretic mobility shift assay (EMSA). The results showed that the cellular proliferation of curcumin group (OD value 0.640 +/- 0.073) was decreased, as compared with that of the CML cells group (OD value 0.856 +/- 0.083, P <0.01). The expression levels of STAT5 mRNA in CML cells group (integral ratio of OD 1.782 +/- 0.156) were significantly greater than that in the normal control group (integral ratio of OD 0.289 +/- 0.025, P <0.01). The expression of STAT5 mRNA in curcumin group (integral ratio of OD 1.398 +/- 0.126) was significantly decreased as compared with that in the CML cells group (P <0.01). The activation of STAT5 was significantly increased in CML cells group (gray value 5323.375 +/- 515.640) as compared with that in the normal control group (gray value 2943.000 +/- 273.377, P <0.01). The activation of STAT5 of curcumin group (gray value 4331.750 +/- 398.035) was significantly decreased as compared with that of CML cells group (P <0.01). It is concluded that the cellular proliferation and the expression of STAT5 mRNA are increased in the primary CML cells. The activation of STAT5 in primary CML cells is markedly enhanced. STAT5 signaling pathway may be involved in the proliferation of primary CML cells. Curcumin can inhibit the cellular proliferation and the expression of STAT5 mRNA, and down-regulate the activation of STAT5 in primary CML cells. Curcumin may be used in treatment of leukemia.