Acute lymphoblastic leukemia (ALL) is the most common pediatric cancer. In utero and postnatal exposures to various carcinogens may play a role in the etiology of this disease. N-acetyltransferases, encoded by the NAT1 and NAT2 genes are involved in the biotransformation of aromatic amines present in tobacco smoke, environment, and diet. Their rapid and slow acetylation activity alleles have been shown to modify the risk to a variety of solid tumors in adults. To investigate the role of NAT1 and NAT2 variants as risk-modifying factors in leukemogenesis, we conducted a case-control study on 176 ALL patients and 306 healthy controls of French-Canadian origin. Slow NAT2 acetylation genotype was found to be a significant risk determinant of ALL (odds ratio, 1.5; 95% confidence interval, 1.0-2.2) because of overrepresentation of the alleles NAT2*5C and *7B and underrepresentation of NAT2*4. Besides a slight increase in NAT1*4 allele frequency among cases, no independent association of NAT1 acetylation genotypes and ALL risk was observed. However, the risk associated with NAT2 slow acetylators was more apparent among homozygous individuals for NAT1*4 (odds ratio, 1.9; 95% confidence interval, 1.1-3.4). When NAT2 slow acetylators were considered together with the other risk-elevating genotypes, GSTM1 null and CYP1A1*2A, the risk of ALL increased further, which showed that the combination of these genotypes is more predictive of risk then either of them independently. These findings suggest that leukemogenesis in children is associated with carcinogen metabolism and consequently related to environmental exposures.