BACKGROUND AND PURPOSE: Juvenile- or adult-onset forms of severe 5,10-methylenetetrahydrofolate reductase (MTHFR) deficiency manifesting as complicated hereditary spastic paraplegia have rarely been described. METHODS: Two siblings with mental retardation developed a progressive spastic paraparesis in their late teens. Their diagnostic assessment included extensive neurophysiologic, neuroimaging and metabolic studies. RESULTS: Brain magnetic resonance imaging showed occipital white matter alterations, and electromyography documented a mixed polyneuropathy. Severe hyperhomocisteinemia (>150 mumol/L) associated with the characteristic amino acid profile suggested a diagnosis of severe MTHFR deficiency, confirmed by MTHFR direct sequencing. Treatment with betaine and vitamins benefitted patients' symptoms and diagnostic features. CONCLUSIONS: Severe MTHFR deficiency can be a rare, treatable cause of autosomal recessive complicated hereditary spastic paraplegia. Its screening should be part of the diagnostic flowchart for these disorders.