OBJECTIVE: To investigate the relationship between the thiopurine methytransferase (TPMT) gene polymorphisms and its enzymatic activity, and to clarify the significance of TPMT activity and gene polymorphisms on individualized therapy with thiopurines. METHODS: The TPMT activity and gene polymorphisms were determined in an unrelated population of 250 Chinese healthy blood donors, 100 cords blood and 280 patients with acute leukemia. The TPMT genotyping assay was based on polymerase chain reaction (PCR), restriction digestion of PCR products, denaturing high-performance liquid chromatography (DHPLC) and SNaPshot sequencing and direct DNA sequencing in the TPMT exon 5 (G238C), TPMT exon7 (G460A) and TPMTexon10 (A719G). Erythrocyte TPMT activity was measured by high-performance liquid chromatography (HPLC). RESULTS: The frequency of TPMT polymorphism in 250 Chinese healthy blood donors, 100 cords blood and 280 patients with acute leukemia was low (3.5%), and all the varied alleles were TPMT* 3C (exon 10A719G). All of them were TPMT* 1/TPMT* 3C heterozygote. The TPMT activity was between 6 and 12 U. The activity in 95.1% was more than 12 U (13 - 32 U), while the activity in others (4.9%) was 6 - 12 U. TPMT activity and genotype were concordant. Of 630 subjects evaluated, TPMT activity of heterozygous individuals in Chinese healthy blood donors, cords blood and acute leukemia patients were 9.1 U, 9.3 U and 9.07 U, respectively, significantly lower than that in general population (17.6 U, 17.67 U and 18.6 U, respectively). In the samples analyzed, ten subjects with heterozygous phenotypes (6/15 acute leukemia children and 4/16 healthy blood donors and cords blood) did not have TPMT* 2, TPMT* 3A or TPMT* 3C. Therefore, other factors may affect on TPMT activity. CONCLUSION: TPMT gene polymorphisms and its activity were concordant. The heterozygotes had low TPMT activity. Therefore, detection of TPMT genotype and its activity is useful. These findings hold a promise of improving the safety and efficacy of thiopurines therapy.