 TPMT is a key enzyme in the metabolism of thiopurines. TPMT polymorphisms have been described and are associated with a decrease activity of such enzyme. Therefore, a higher risk of developing toxicity is present in patients requiring these drugs, which are indicated in acute lymphoblastic leukemia, as well as, immunosuppressors after organ transplantation. The frequency of heterozygotes polymorphisms ranges from 3 till 12 , in different populations. Homozygous patients have a lower frequency, estimated 1 in 300 individuals. The frequency of such polymorphisms in mestizos mexican population has not been analyzed, and we considered important to determine this frequency in healthy and patients requiring thiopurines, particularly acute lymphoblastic leukemia. 