Several polymorphic glutathione S-transferase (GST) enzymes are involved in the detoxification of active metabolites of many potential carcinogens and may therefore be important in modulating susceptibility to cancers. GSTM1 and GSTT1 are polymorphic, and the null alleles result in a lack of corresponding enzyme activities. Previous studies demonstrated that the GSTM1 and GSTT1 null genotypes correlated with an increased risk of developing some cancers. In this study, we determined GSTM1 and GSTT1 polymorphisms in a population of 131 healthy controls from the south of Iran, 46 patients with colorectal cancers, and 42 patients with gastric cancer. The gastric cancer risk statistically increased due to the GSTM1 null genotype (odds ratio (OR)=2.3, 95% confidence interval (CI): 1.15--4.95). On the other hand, the GSTT1 null genotype in gastric cancer and null genotypes of GSTM1 and GSTT1 in colorectal cancer were not statistically significant. Moreover, individuals showing the GSTM1 and GSTT1 null genotypes might exhibit a greater predisposition to gastric (OR=3.31, 95% CI: 1.14--9.57) and colorectal (OR=2.73, 95% CI: 0.94--7.95, P=0.07) cancers.