Glutathione S-transferases (GSTs) are enzymes involved in the detoxification of several environmental mutagens, carcinogens and anticancer drugs. GST polymorphisms resulting in decreased enzymatic activity have been associated with several types of tumors. Using PCR, GSTT1 and GSTM1 genotypes were determined in 40 adults with acute leukemia and 20 age and sex matched controls. In acute leukemia combined, a slightly higher proportion of cases displayed the GSTT1 and GSTM1 null genotypes than controls , but not statistically significant . The null genotype of GSTM1 occurred in 25(62.5%) and 9(45%) of their corresponding controls (odds ratio (OR) 2.037, 95% confidence interval (CI) 0.686-6.052). 12(30%) of cases carried GSTT1 null genotype and 4(20%) in controls (OR 1.714, 95% CI 0. 47-6.21). AML showed the strongest association with GSTM1 null genotype, 15(75%) of AML cases were null compared with 9(45%) in controls (OR 3.66, 95% CI 1.01-14.03), a slightly higher proportion of AML cases (25%) displayed GSTT1 null genotype compared with controls (20%), although the difference was not statistically significant (OR 1.33, 95% CI 0.3-5.926). In ALL 10(50%) were null in GSTM1 yielding (OR 1.22, 95% CI 0.353-4.235) and 7(35%) of cases were null in GSTT1 yielding (OR 2.154, 95% CI 0.516-9.00) , so no significant association was found between either GSTM1 or GSTT1 null genotype and ALL. The interaction between both genotypes was also studied , in AML increased risk estimates were observed when either allele was null , it was found a strong association between AML and (GSTM1 null/ GSTT1 present) genotype (OR 18, 95% CI 1.9-171.9) and also between AML and (GSTT1 null/GSTM1 present) genotype (OR 18, 95% CI 1.25-260.9) p < 0.05. In ALL , there was no association between ALL and both genotypes ( GSTM1 null/GSTT1 present) (OR 1.5, 95% CI 0.34-6.53) and (GSTT1 null/GSTM1 present) (OR 3.0, 95% CI 0.41-21.89). A significant association was found between smoking and genotype (GSTT1 null/GSTM1 present) (OR 18, 95% CI 1.3- 255.8)