The frequency of the 677C>T mutation in the methylenetetrahydrofolate reductase gene in Mexico is one of the highest worldwide. Some studies have shown that both the homozygous state of this mutation and a high homocysteine concentration are associated with congenital heart disease. The aim of this study was to determine whether this association exists in the Mexican population. Genotypes were analyzed in 60 patients with congenital heart disease and in their mothers, and the levels of homocysteine were determined in the latter group. The genotypes were compared with those of a control group (n=62) and of their mothers. All the possible mother-child genotype combinations were also compared. There were no significant differences in allele or genotype frequencies between the patients with congenital heart disease and the controls or their respective mothers (P>.05). Although no significant differences were observed when the homocysteine concentrations in the presence of the CC or the TT genotype were compared, a clear trend was observed (P=.0621). We found no significant differences in homocysteine concentrations in relation to folic acid intake. The study cases and controls did not differ in terms of the possible combinations of mother-child genotypes. The frequencies obtained were consistent with those reported for Mexico. No significant differences were found between groups. Nor did we find any association between TT mutations in both the mother and child and hyperhomocysteinemia. There was no evidence of an association between any of the mother-child genotype combinations and congenital heart disease. Similar studies with larger numbers of patients are required to confirm or refute some of the trends observed in this report. Copyright © 2011 Sociedad Española de Cardiología. Published by Elsevier Espana. All rights reserved.