Objective To study the mechanism of CD4 + CD25 + forkhead box P3 (Foxp3) + regulatory T cells inhibiting the tumor immunity in colorectal cancer.Methods Thirty-two cases of colorectal carcinoma,21 cases of polypus,and 18 cases of normal tissues were collected.T he immunohistochemistry was used to measure the number of Foxp3 + regulatory T cells and interleukin (IL)-10 positive cells in above tissues,and reverse transcription-polymerase chain reaction (RT-PCR) was done to dectect the expression of Foxp3 and signal transducers and activators of transcription 3 (STAT3) mRNA in different tissues.The relationship between the above indexes and clinicopathological features was analyzed.Results The expression rate of positive cells of Foxp3 and IL-10 in colorectal cancer was respectively 45.8% and 68.8%,significantly higher than in the polypus tissues (13.6% and 1.8%) and normal tissues (9.5%and 23.8%) (P <0.01 for all).The relative expression amount of Foxp3 and STAT3 mRNA in tumor tissue was respectively 0.644 ± 0.059 and 0.343 ± 0.036,which was significantly increased as compared with polypus (0.322 ±0.020 and 0.212 ±0.028) and normal tissue (0.309 ±0.016 and 0.202 ±0.021)(P <0.01 for all).The Foxp3 mRNA expression in colorectal cancer tissues was positively correlated with STAT3 (r =0.526.P < 0.05).The Foxp3 protein expression wass positively correlated with IL-10 (r =0.314,P < 0.05).The expression of Foxp3 in cancer patients was visibley correlated with the histological grade,lymph node metastasis and TNM stage (P < 0.05),and there was no visible correlation with age and gender (P > 0.05).Conclusion CD4 + CD25 + Foxp3 + regulatory T cells inhibit tumor immunity and induce tumor immune escape through the expression of Foxp3 and some other related inhibitory cytokines,and finally promote the tumor progression. Key words: Regulatory T cell; Forkhead box P3; Signal transducers and activators of transcription 3 ; Colorectal cancer; Tumor immunity