When the study of Cong et al. [33] was excluded, the I2 value dropped and no significant association was observed between the combined effects of GSTM1 present/null and GSTT1 present/null polymorphisms and CRC risk in population-based studies (− − vs. + −: OR = 1.13, 95% CI: 0.78–1.65, I2 = 54.4%; − − vs. − +: OR = 0.88, 95% CI: 0.65–1.19, I2 = 55.3%). This evidence concerns the gene GSTM1 and colorectal carcinoma.