Disturbances in methyl-carbon metabolism, which result in hyperhomocysteinemia, have been associated with schizophrenia. Homozygosity for the T677 allele of the methylenetetrahydrofolate reductase (MTHFR) gene, which encodes for a thermolabile enzyme associated with hyperhomocysteinemia, has been found to be increased in schizophrenic patients. We have investigated whether plasma homocysteine concentration and the frequency of C677T MTHFR variant were increased in schizophrenic inpatients of a psychiatric hospital (n=210) compared with controls (n=218). There were no significant differences in plasma homocysteine concentrations between the schizophrenia and the control group. The distributions of T allele and TT genotype frequencies were similar in both groups (40% and 15%). These results show that impaired homocysteine metabolism is unlikely in schizophrenia.