Patients with Down syndrome have been found to have characteristic in vivo and in vitro methotrexate toxicity. The in vitro methotrexate toxicity characteristic of Down syndrome can be diminished by the in vivo administration of supplemental high doses of folic acid. A possible explanation for the increased sensitivity to methotrexate which has been documented in patients with Down syndrome may be due to imbalances in nucleotide pools which result from a gene dosage effect and to greater methylation demands. Supplemental folic acid may be beneficial by virtue of a down-regulation of excess gene activity and may also provide needed monocarbons.