The enzyme glutathione S-transferase Mu 1 (GSTM1) is encoded by the GSTM1 gene. Polymorphisms in GSTM1 affect the detoxifying function of the enzyme variants. This forms the basis of the debate about the impact of the GSTM1 null/present genotype on colorectal carcinoma risk. We tested the potential influence of GSTM1 polymorphisms on the development of colorectal cancer. DNA extracted from 83 samples taken from patients that were previously diagnosed as having colorectal carcinoma and from 35 control subjects who did not have colorectal carcinoma were amplified. GSTM1 genotypes were determined by DNA sequencing. The current study revealed that the majority (69/83, 83%) of colorectal cancer cases harbored the null genotype (GSTM1*0/*0), and the remaining 14 (17%) cases harbored either the GSTM1wt/wt or the GSTM1wt/*0 genotype. In contrast, among the control cases, 23 (65%) had the null genotype (GSTM1*0/*0) and 12 (35%) had either the GSTM1wt/wt or the GSTM1wt/*0 genotype. The current report emphasizes the impact of the GSTM1 null genotype on the increased risk of colorectal carcinoma in Saudi Arabia.