We investigated the frequency of the GSTM1 genotypes in 70 Brazilian patients with oral squamous cell carcinoma (OSCC) and 82 age-sex matched controls. The GSTM1 genotypes were studied by PCR-based methods. The frequency of male patients with OSCC and null for the GSTM1 (70.5%) was statistically different from the male patients from the control group (48.5%) (Odds Ratio, OR=2.53, 95% CI=1.22-5.24, P<0.05). The frequency of the GSTM1 null genotype (0/0) in the group with OSCC (65.7%) was statistically different from the controls (48.7%) (OR=2.01, 95% CI=1.04-3.88, P<0.05). The prevalence of GSTM1 deficiency (null) was significantly higher for patients with oral cancer of the floor of the mouth (OR=3.67, 95% CI=1.11-12.11, P<0.05). In conclusion, the GSTM1 null genotype may increase the risk for OSCC development.