OBJECTIVE: There have been a kind of transmethylation theory that high homocysteine serum concentration affects schizophrenia by neurotoxic mechanism and clinical reports that some schizophrenic patients with high homocysteine were improved by high folate ingestion. This study was done to confirm previous research results and find the clinical characteristics of schizophrenia showing high serum homocysteine and low folate. METHOD: We compared the serum levels of homocysteine, folate and vitamin B12 level between 234 schizophrenic patients(male 99, female 135) group and 234 normal controls(male 99, female 135) group. The subjects of two groups were age and sex matched. The evaluated clinical characteristics items were sex, age, onset of disease, hereditary loading, disease course, hallucination and subtype of schizophrenia. RESULTS: 1) Homocysteine level of the schizophrenia group was significantly higher than the normal control group and folate level of the schizophrenia group was significantly lower than the normal control group. Homocysteine level was more negatively correlated with folate level in the schizophrenia group than the normal control group. 2) The percentage of high homocysteine(above 12.46umol/L;90 percentile of normal control) was 33.8% of schizophrenia patients and 51.5% of male schizophrenia. The percentage of low folate(below 3.8nM/L;bottom tertile of normal control) was 66.2% of schizophrenia. 3) In low folate group and not-low folate group, schizophrenia showed significantly higher homocysteine level than normal control. Especially, low folate schizophrenia group showed significantly higher homocysteine level than low folate normal control group. CONCLUSIONS: Some schizophrenia patients with high serum homocysteine may be genetic defector and having low folate serum level. In that case, folate ingestion could be a good management for clinical improvement.