Epidemiological studies have demonstrated that the glutathione S-transferase (GST) null genotype is related to an increased susceptibility to diseases associated with several types of cancer. This study investigated the risk of developing gastric or colorectal cancer related to the GST polymorphisms in the Korean population. Genotyping was performed on the subjects including 102 subjects with gastric cancer (GC), 172 subjects with colorectal cancer (CRC), and 200 healthy subjects as the control group. As a result, the GST theta1 (GSTT1) null genotype was associated with a 1.5-fold increased risk of developing CRC (95 % confidence interval (CI)=1.1-2.3, P=0.04). A significant association was found between the CRC and the combined genotypes of the GST mu1 (GSTM1) and GSTT1 (odds ratio02.1, 95 % CI=1.1-4.2, P=0.03). There were no significant associations between the GST polymorphisms and the GC subjects. These results suggest that the GSTT1 null genotype is associated with an increase in the risks of CRC in the Korean population.