Objective: To investigate the association between polymorphisms in the human CYP2A6 gene, CYP2A6 enzyme activity and the influence of cigarette smoking in a Thai population. Methods: Coumarin (5 mg capsule) was administered to 194 healthy Thai subjects. Genetic variation of the CYP2A6 gene was identified using PCR methods. The excreted dose of 7-hydroxycoumarin (as a percentage of the urine concentration) 2 h after administration was calculated as an index of coumarin metabolism. Results: The frequencies of CYP2A6 alleles1A,1B,4C,7,8,9 and10 were 34.0, 35.3, 9.3, 6.4, 0.5, 12.1 and 2.4%, respectively. Of the 194 subjects tested, the number (percentages) of Thai participants classified as ultra-rapid, extensive, intermediate and poor metabolizers were 8 (4.1%), 159 (82.0%) 22 (11.3%) and five (2.6%), respectively. Conclusion: A relationship between the interindividual differences in coumarin metabolism and genetic polymorphisms of the CYP2A6 gene was observed.