The therapeutic effects of haemodialysis were evaluated in schizophrenic patients using a double blind procedure. Twelve patients were diagnosed as acute schizophrenics according to Feighner's criteria for psychopathology. After obtaining informed consent, the patients were randomly assigned to active haemodialysis (AD) or sham dialysis (SD). An 8 days "drug washout period" was followed by AD or SD treatment of 4 weeks (two 5 hours sessions per week) and psychopathological evaluations were performed regularly in a blind fashion using the Brief Psychiatry Rating Scale (BPRS) and the Comprehensive Psychopathological Rating Scale (CPRS). Nine of the 12 patients were improved by both extracorporeal procedures with or without active dialysis. No significant difference appeared however between both groups in the rate and degree of improvement of nuclear symptoms of schizophrenia. Nevertheless, AD was significantly more efficient in relieving affective symptomatology, suggesting the potential involvement of some endogenous dialysable substance (s) in the pathogenesis of mood disturbances in schizophrenia.