Subacute combined degeneration (SCD) is a metabolic disease caused by deficiency of vitamin B12. Rarely, it could be associated with genetic problem. An old male presented with progressive both hands weakness. Laboratory study showed deficiency of vitamin B12, but the cause was not clear. We performed a genetic study and methylenetetrahydrofolate reductase (MTHFR) C677T homozygous polymorphism with 30% of normal enzyme activity was confirmed. This case suggests SCD may occur in association with a genetic problem with MTHFR C677T polymorphism.