BACKGROUND/AIMS: Glutathione S-transferases (GST) are involved in the detoxification of many potential carcinogens and appear to play an important role in the protection from carcinogens. The association between different GSTM1 genotypes and gastric cancer risk is still controversial. The aim of this study was to determine the association between the GSTM1-null genotype and gastric cancer risk. METHODOLOGY: A retrospective, hospital-based case-control study of 123 primary gastric cancer patients and 121 healthy controls was conducted to evaluate the presence or absence of the GSTM1 gene from peripheral blood samples by a PCR-based method. RESULTS: The frequency of the GSTM1-null was 59.3% in the gastric cancer group and 45.5% in the control group. An increase in risk for gastric cancer was found among carriers of GSTM1-null genotype. The odds ratio (OR) was 1.752, with 95% confidence interval (CI) = 2.963 approximately 1.035. CONCLUSIONS: GSTM1-null polymorphism was associated with genetic susceptibility of gastric cancer in Taiwan.