Background: Recent studies have shown that a common mutation (nucleotide 677 C→T) in the methylenetetrahydrofolate reductase (MTHFR) gene may contribute to a mild rise in plasma homocysteine levels and increase the incidence of coronary artery disease. Therefore, this study was designed to further investigate whether the effects of MTHFR 677 C to T mutation, plasma homocysteine, serum vitamin B12, and folate can influence restenosis after successful coronary stenting. Methods and Results: We investigated 260 patients each with a lesion after successful coronary stent placement. All patients received a repeated angiography after 6 months, or earlier if clinically indicated. Angiographic in-stent restenosis (ISR) was defined as ≥50% diameter stenosis at follow-up. Genotyping for MTHFR was based on a polymerase chain reaction technique. Also fasting plasma homocysteine, vitamin B12, and folate levels were measured at the same time. The ISR rate was higher among the patients with the TT genotype than in those with the non-TT genotypes (64.0% versus 32.9%, P = 0.002). There was no significant difference in plasma homocysteine levels among patients with the TT genotype and patients with the non-TT genotypes (15.9 ± 7.6 versus 15.5 ± 6.6 μmol/L, P = 0.75). However, among the patients with the TT genotype, those with higher plasma homocysteine levels (≥12 μmol/L) demonstrated a significantly higher ISR rate (75.0% versus 33.5%, P = 0.001). Logistic regression analysis revealed that the combined presence of the MTHFR TT genotype and lower than average serum vitamin B12 (≥550 pg/mL) resulted in the most significant difference in the risk of ISR (OR = 3.57, CI = 1.51–8.46, P = 0.004; OR = 2.36, CI = 1.35–4.15, P = 0.003). Conclusions:MTHFR 677TT polymorphism and low serum vitamin B12 each individually increased the risk of coronary ISR. Furthermore, the combination of these parameters resulted in a greater increase in risk. © 2006 Wiley-Liss, Inc.