Severe deficiency of methylenetetrahydrofolate reductase (MTHFR) results in homocystinuria, with a variety of neurological and vascular complications, and sometimes death in the first year of life. MTHFR (EC 126.96.36.199) catalyses the synthesis of 5-methyltetrahydrofolate (5-methylTHF) which is required for homocysteine remethylation to methionine. Mthfr−/− mice are a good animal model of severe MTHFR deficiency in humans. They have marked hyperhomocysteinaemia and a high rate of mortality in the neonatal period. We attempted to rescue Mthfr−/− mice from postnatal death by treating their Mthfr+/− mothers with mefolinate (a synthetic form of 5-methylTHF, dissolved in their drinking water) or with a folic acid-enriched diet throughout pregnancy and lactation. We monitored pups' vitality and body weights until 3 weeks of age. The majority of Mthfr−/− pups from the control groups died during the first week of life. Body weights of −/− pups from control groups were significantly less than those of their Mthfr+/− and Mthfr+/+ littermates. Mefolinate treatment significantly improved survival rates (64% survival) in the −/− pups and improved morphology of the cerebellum. Folic acid supplementation did not affect the survival rate or body weights of the −/− pups. Our study suggests that MTHFR is important for postnatal growth and vitality, and that 5-methylTHF deficiency contributes to the high postnatal mortality. Mefolinate may be a good candidate drug for treatment of severe MTHFR deficiency.