We have examined the transmission frequencies of the methylenetetrahydrofolate reductase (MTHFR) 677 T and C alleles from heterozygous parents to children with Down syndrome (trisomy 21) in 202 Caucasian families. Our results indicated that the MTHFR 677T allele was transmitted to children with Down syndrome at a significantly higher rate than would be expected based on Mendelian inheritance patterns, and the C allele was transmitted at a significantly lower rate (P < 0.009). Transmission frequencies were also examined independently for maternally and paternally transmitted alleles to assess potential parent-of-origin effects. Because the vast majority of conceptions with trisomy 21 end in pregnancy loss, we questioned whether the observed preferential transmission of the T allele to this population of liveborn infants with Down syndrome could reflect a survival advantage. A plausible biochemical interpretation of these results is presented based on a maternal-fetal MTHFR 677T allele interaction in the context of the constitutive overexpression of three copies of the cystathionine beta synthase gene in the trisomy 21 fetus. Published 2002 Wiley-Liss, Inc.