We studied the role of cytochrome P4501A1 (CYP1A1 Val/Val) genotypes in the etiology of acute lymphoblastic leukemia (ALL) in adult Mexican patients. Distributions of CYP1A1 Val/Val genotypes in peripheral blood DNA samples from 136 healthy controls and 136 adult patients with ALL were evaluated. There was an increased frequency of the CYP1A1 Val/Val genotype among ALL patients, showing a significant association between this genotype and the risk of developing ALL.