Two representative genetic variants of the MDR1 gene, 3435C>T and 2677G>T/A, show wide interethnic differences in its genetic polymorphism. In this study, the authors evaluated the genetic polymorphisms of MDR1 and directly compared MDR1 haplotype profiles of the Korean and Vietnamese populations. The 3435C>T and 2677G>T/A variations were analyzed in 632 Koreans and 142 Vietnamese using pyrosequencing. The allelic frequencies of 3435C>T did not significantly differ between the Korean (39.3%) and Vietnamese (36.6%) groups. However, the frequencies of mutant alleles at 2677 locus (T or A allele) showed a significant difference between Koreans (56.2%) and Vietnamese (41.9%), as the frequency of 2677A allele in the Korean subjects (17.1%) was much higher than that of the Vietnamese subjects (6.3%). Linkage analysis revealed that 2677A allele is closely linked to 3435C allele. The frequency of 2677A-3435C haplotype in Koreans was 15.4%, which was significantly higher than that found in Vietnamese subjects (6.3%). In conclusion, the frequencies of MDR1 variants and haplotype profiles showed significant differences between the Korean and Vietnamese populations, especially with respect to the 2677G>T/A variants. Because the 2677A allele was recently found to be functional in vivo and was detected at a high frequency in Koreans, the genotyping of this variant is necessary for pharmacogenetic studies of MDR1 in this population. In addition, by virtue of strong linkage disequilibrium, 2677A-3435C haplotype may help improve the predictability of MDR1 genetic polymorphism for MDR1 functional changes.