In the diet, folate exists predominantly in the form of polyglutamates. Before absorption, these polyglutamates must be deconjugated to monoglutamates by the enzyme folylpoly-gamma-glutamate carboxypeptidase (FGCP), which is located in the jejunum. Recently, a H475Y polymorphism in the glutamate carboxypeptidase II (GCPII) gene, encoding the FGCP enzyme, was reported to be associated with decreased plasma folate and increased plasma homocysteine (tHcy) levels. Low folate and elevated tHcy levels are risk factors for neural tube defects (NTD). Therefore, we examined whether this polymorphism is associated with NTD risk and plasma folate, erythrocyte folate and plasma tHcy levels in 96 NTD patients, 113 mothers, 97 fathers and 101 controls. This variation was associated with increased plasma folate (P < 0.04) and tended to be associated with decreased plasma tHcy (P < 0.09). It was not associated with erythrocyte folate or the risk for NTD. The H475Y polymorphism in the GCPII gene may increase the deconjugation activity of the FGCP enzyme, resulting in an increased absorption of folate in the body, as reflected by the increased plasma folate and decreased plasma homocysteine concentrations.