the present study evaluates genetic polymorphisms of three glutathione S-transferases (GSTM1, GSTT1and GSTP1) in patients with synchronous malignant colorectal tumors and the association of synchronous colorectal cancers with other cancers.,from 420 patients with a colorectal cancer admitted to our hospital between 2005-2012, we selected for genetic analysis 20 patients with multiple synchronous malignant colorectal tumors and 9 patients with asynchronous association of colorectal cancer with another cancer. We searched for GST genotypes, comparing the results with controls.,the genetic analysis was possible only in 19 patients with colorectal synchronous cancers and 9 patients with asynchronous association of colorectal cancer with another cancer; we found a statistically significant difference for null GSTM1 genotype frequency between these patients and the control group; we found no differences regarding the frequency of null GSTT1 genotype and Ile105Val polymorphism of GSTP1 in patients with synchronous cancers compared with the control group.,in our study we found the null GSTM1 genotype as a risk factor for multiple colorectal synchronous cancers and for an association of synchronous colorectal with other cancers