The involvement of phase I and II enzymes is well documented in the metabolism of a wide range of drugs and xenobiotics. Single-nucleotide polymorphisms (SNPs) of these enzymes are also known to alter their protein expression and function. Moreover, genetic susceptibility and environmental exposure have been proposed to be an etiology of cancer. We hypothesized that polymorphisms of these enzymes might affect the risk of childhood acute lymphoblastic leukemia (ALL). CYP 1A1, CYP 3A4*1B, CYP 3A5*3, CYP 3A5*6, GSTM1, and GSTT1 polymorphisms were genotyped by using PCR-RFLP in 107 children with ALL and 320 healthy controls. Allele and genotype frequencies of each of the SNPs were compared between two groups. It was found that the allele frequencies of CYP 1A1*1, *2A, *2B, and *4 were not different between cases and controls. CYP 3A4*1B allele frequency was only 0.8% and 0.9% in ALL and controls, respectively. CYP 3A5*1/*1, *1/*3, and *3/*3 genotype frequencies showed no statistically significant difference between patients and controls. CYP 3A5*6 was not detected in our population. The GSTM1 null genotype was significantly increased in children with ALL (OR 1.7; 95% CI, 1.0, 2.7). In contrast, the GSTT1 null genotype did not show this effect. Our data thus demonstrate that the GSTM1 null genotype might increase the risk of childhood ALL in a Thai population.