Methylenetetrahydrofolate reductase (MTHFR) deficiency is a rare autosomal recessive disorder of homocysteine metabolism. Neurologic symptoms usually appear in early life with developmental delay, microcephaly, hypotonia, seizures, apnea and coma. We report on a 56 year-old woman with a history of depression and psychosis, who exhibited acute paraplegia, coma and leukoencephalopathy. High plasma homocysteine with low methionine levels suggested a defect in homocysteine re-methylation. Severe MTHFR deficiency was found, with a new mutation in the MTHFR coding gene. Under treatment consisting of high doses of folinic acid, betaine monohydrate, beflavin and cobalamin, plasma homocysteine normalized with parallel psychiatric, cognitive and brain MRI improvement. Paraplegia remained unchanged. In conclusion, MTHFR deficiency should be considered at any age in patients with an unexplained neuropsychiatric disorder. A simple screening test, such as plasma homocysteine determination, should be performed in order to start treatment before irreversible spinal cord damage has occurred.