Objective To investigate the correlations of Mycoplasma penetrans(MPe)infection with the differentiation.invasiveness and metastasis of stomach and colorectal calFcinomas.Methods Sixty five patients with stomach carcinoma,57 patients with colorectal carcinoma and 80 healthy individuals as controls were enrolled in this study.MPe was isolated and then confirmed by PCR.χ2 test was performed to analyze the correlations of MPe infection with the differentiation,invasiveness and metastasis of carcinoma.Results The rate of MPe isolated from stomach carcinoma group(41/65,63.1%)was significantly higher than that from stomach ulcer group(χ2=38.2,P＜0.01).The rate of MPe isolated from eolorectal carcinoma group (1/20,5%)was also significantly higher than that from colorectal polyps group(χ2=21.2,P＜0.01).The proportion of poor differentiation and the invasiveness in MPe positive stomach carcinoma group were significantly higher than those in MPe negative group(χ2:33.4 and 25.0.P＜0.01).The proportion of poorly differentiation and lymphatic metastasis(N3)in MPe positive colorectal carcinoma group were significantly higher than those in MPe negative group(χ2=34.4,P＜0.01).Conclusion Differentiation,invasiveness and metastasis are highly correlated with MPe infection in stomach and colorectal carcinomas.