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Isoniazid (INH) is one of the most common drugs for tuberculosis (TB) treatment and INH acetylation catalyzed by non-inducible hepatic enzyme arylamine N-acetyltransferase type 2 (NAT2). The isoniazid acetylation rates, which depend on NAT2 genotypes, is constant in an individual but can changes between patients. Phenotypic groups can be classified based on the genotype: slow, intermediate, and rapid acetylators. This study was performed to identify the relation between NAT2 gene polymorphisms and plasma INH concentrations among the different genotypes of Vietnamese tuberculosis patients. Blood samples of 136 adult TB patients treated with INH were collected and genotyped for NAT2 gene polymorphisms using Sanger sequencing. Two-hour post-dosing INH plasma concentrations were determined by high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS). Among the 136 patients genotyped, there were 43 (31.62 %), 58 (42.65 %), and 35 (25.74 %) of slow, intermediate, and rapid acetylation phenotypes, with two-hour post dosing INH plasma concentrations of 3.4, 2.7, and 2.2 μg/ml, respectively. The differences in INH concentrations among the three genotypes were significant (P < 0.05). Genotyping of TB patients from Vietnam for NAT2 gene polymorphism revealed that 48 percent of the study population comprised slow acetylators. Two-hour INH levels were significantly different among CC and TT homozygous genotypes of NAT2(C282T), as 2.7 μg/ml and 3.9 μg/ml, respectively. This suggests that NAT2(C282T) could play a role in INH metabolism in TB patients. Methods: Blood samples of 136 adult TB patients treated with INH were collected and genotyped for NAT2 gene polymorphisms using Sanger sequencing. Two-hour post-dosing INH plasma concentrations were determined by high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS). Results: Among the 136 patients genotyped, there were 43 (31.62%), 58 (42.65%) and 35 (25.74%) of slow, intermediate and rapid acetylation phenotypes with two-hour post dosing INH plasma concentration of 3.4, 2.7 and 2.2 μg/ml, respectively. The differences in INH concentrations among the three genotypes were significant (P