We investigated the prevalence of a genetic variation in the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene (C677T) using polymerase chain reaction techniques in a sample of 500 general Thai population and among 40 unselected Thai patients with an objectively confirmed history of deep vein thrombosis (DVT). The prevalence of the mutated homozygous and heterozygous C677T MTHFR genotype in the group of 500 healthy Thai population was 1.4 and 25.6%, respectively (allele frequency of 14.2%). Of the 40 patients studied, none were homozygotes and 15% were heterozygotes for the C677T MTHFR gene mutation (allele frequency of 7.5%). There was no significant difference in genotype frequency between patients and control groups (p = 0.09). Odds ratios for the probability of the C677T MTHFR gene mutation in the patient versus control group were 0.49 (95% CI 0.21–1.12). These data indicated that the C677T MTHF gene mutation was not associated with DVT in the Thai population. The lower frequency of the C677T MTHFR gene mutation in our Thai population compared with reports from other studies suggests a wide heterogeneity in the 677T MTHFR genotype frequencies of the different ethnic populations even among Asians.