Schizophrenic patients generally appear to have a disturbed single-carbon metabolism. Methionine and homocysteine are intermediary metabolites in this metabolic system. In a case-control study of the cerebrospinal fluid, a majority of the patients had elevated methionine and a smaller subgroup had elevated homocysteine. Elevated homocysteine is often explained by folate dependency due to mutations in the gene for methylenetetrahydrofolate reductase (MTHFR). A most encouraging feature of single-carbon metabolism is its potential modification by natural means, such as B-vitamins and antioxidants. The findings point to a new area of schizophrenia research: the role of nutrients and antioxidants for rational prevention and treatment.