Free radicals may be involved in the pathogenesis of tardive dyskinesia (TD). Superoxide dismutase (SOD), a critical enzyme in the detoxification of superoxide radicals, was found to be abnormal in TD. To examine the blood SOD levels in schizophrenic patients with and without TD, and the relationships between SOD levels and tardive dyskinesia symptoms in TD patients, 45 physically healthy patients with TD who met DSM-III-R criteria for schizophrenia were compared with 45 schizophrenic patients without TD, as well as with 50 age-, sex- and smoking-matched normal controls. The severity of TD was assessed using the abnormal involuntary movement scale (AIMS). The psychopathology of patients were assessed by the Positive and Negative Syndrome Scale (PANSS). Blood SOD levels were measured by radioimmunometric assay (RIA). The results showed that the patients with TD had lower concentrations of blood SOD than those without TD, but had higher blood SOD levels than the normal controls. In the patients with TD, AIMS total score was inversely correlated with SOD levels. Our data support the view that free radicals may be involved in the pathophysiology of TD. There may exist a relationship between the free radical metabolism and the severity of dyskinesia of TD patients.