Folic acid intake reduces the risk of neural tube defects (NTDs). Although the 677C-->T mutation in the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene is a risk factor for NTDs, it only partly explains the elevated homocysteine levels in mothers of children with NTDs. We measured vitamin B12, folate and homocysteine in patients with spina bifida (SB), their parents, and in controls, to investigate which other enzymes of homocysteine metabolism might be defective. Because homozygosity for the 677C-->T mutation causes decreased plasma folate and increased red-cell folate (RCF) and plasma homocysteine levels, we excluded individuals homozygous for that mutation. The remaining SB patients and their parents still had lowered plasma folate and elevated total homocysteine levels, and a small subset had decreased vitamin B12 levels. Red-cell folate was the same in all groups, suggesting that dietary folate intake and its uptake was normal. Risk of SB was increased at the 25th percentile of plasma folate and at the 75th percentile of homocysteine values in SB patients and their parents, and at the 5th and 25th percentiles of vitamin B12 in mothers with SB-affected offspring. This underlines the functional importance of homocysteine remethylation to methionine. There was no correlation between vitamin B12 and homocysteine or RCF. In combination with the lowered plasma folate (80-90% 5-methyltetrahydrofolate), our data do not support a major involvement of methionine synthase in the aetiology of SB. Our data rather favour the involvement of genetic variation at loci coding for the formation of 5-methyltetrahydrofolate, such as MTHFR, methylenetetrahydrofolate dehydrogenase or serine hydroxymethyltransferase.