In this study we tested the hypothesis that red blood cell 5-methyltetrahydrofolate, a long-term marker of the folate status, is associated with the severity of coronary artery disease and whether this association is independent of homocysteine, vitamin B12, plasma 5-methyltetrahydrofolate, 5,10-methyltetrahyrofolate reductase C677T genotype, and other cardiovascular risk factors. Two hundred and fifty-one angiographically documented patients aged <70 years with single, double, or triple coronary artery disease were investigated. Red blood cell 5-methyltetrahydrofolate concentrations were significantly decreased with the increasing number of diseased vessels (analysis of variance, P < 0.001). Red blood cell 5-methyltetrahydrofolate was also inversely and significantly correlated with the number of diseased vessels (r = -0.36, P < 0.001). Stepwise multiple regression analysis showed that red cell 5-methyltetrahydrofolate was a strong predictor of number of diseased vessels independent of plasma total homocysteine, 5,10-methyltetrahyrofolate reductase C677T genotype, and all other coronary artery risk factors (beta = -0.002, P < 0.001, r2 = 0.128). The results of this study suggest that low red blood cell 5-methyltetrahydrofolate is associated with the severity of coronary artery disease independent from plasma homocysteine and other cardiovascular risk factors.