Thiopurine S-methyltransferase (TPMT) is a cytosolic enzyme that catalyzes the S-methylation of 6-mercaptopurine and azathioprine. Low-activity phenotypes are correlated with polymorphism in the TPMT gene. Patients with low or undetectable TPMT activity could develop severe myelosuppression when they are treated with standard doses of thiopurine drugs. Since ethnic differences in the TPMT gen polymorphism have been demonstrated worldwide, this remains to be elucidated in Chile. Aim: to investigate the TMPT gen polymorphism in a chilean blood donor population. Patients and Methods: The frequency of four allelic variants of the TPMT gene, *2 (G238C), *3A (G460A and A719G), *3B (G460A) and *3C (A719G) were analyzed in 210 chilean individuals using PCR-RFLP and allele-specific PCR-based assays Results: In total, TPMT variants associated to low enzymatic activity, were detected in 16 subjects (7.6%), who had a heterozygous genotype (*3A in 12; *3C in three and *2 in one subject). No TPMT*3B allelic variant was found. The normal allele (wild-type) was found in 92,4 % of studied individuals. Conclusions: The allele TPMT*3A, in accordance with caucasian populations, is the most prevalent in the chilean population studied. The evaluation of TPMT gen polymorphism could be useful to determinate the clinical efficacy and toxicity of thiopurine drugs.