Conotruncal cardiac defects are partially prevented by maternal folic acid supplementation. However, the biochemical mechanism is unknown. Maternal autoantibodies to folate receptors, previously associated with increased risk for neural tube defects, also may account for this effect. This study aimed to examine the titers of folate receptor-blocking autoantibodies in mothers of children with conotruncal congenital heart defects and to compare them with those in the general population. Serum samples were obtained from 22 women whose pregnancies were complicated by conotruncal congenital heart malformations. Groups of samples were analyzed for autoantibodies against [(3)H] folic acid-labeled folate receptors, quantitative amounts of immunoglobulin G (IgG) and IgM autoantibodies to the folate receptor, and for ability to block-bind folic acid to receptors. No elevated levels of antibodies binding to [(3)H] folic acid-labeled folate receptors were found. No difference was found in antifolate receptor alpha-IgG or IgM median levels between cases (261 vs. 240 mug/mL) and control subjects (773 vs. 924 mug/mL). There was no increased blocking of folic acid binding between cases [0.69 ng/mL; 95 % confidence interval (CI), 0.006-0.01] and control subjects (0.69 ng/mL; 95 % CI, 0.003-0.013). Although epidemiologic evidence suggests that periconceptual folic acid may prevent many conotruncal congenital heart defects, the current study suggests that this effect is unlikely to be explained by the presence of maternal autoantibodies to folate receptor. These data suggest that a strategy of screening women for such autoantibodies will not identify a high-risk group of women to target for supplemental folic acid to prevent congenital heart defects.