A very controversial debate, concerning the treatment of schizophrenic psychoses by means of hemodialysis, started approximately in 1977, when American scientists reported outstanding successes with this method. Until now there is no clear evidence of the underlying principle of effectiveness of this method. According to present experiences there are three theories. Already in 1960 there were reports of successful application of hemodialysis in acute schizophrenic psychoses. The authors, among other things, assumed the filtration of a toxic blood substance to be the therapeutic principle. Later, however, changes in the blood circulation during dialysis, especially the improvement of cerebral perfusion, were seen as the real therapeutic factor. The recently reported findings of abnormal endorphin in the dialysate fluid of schizophrenics, brought back the theory of detoxification. At the same time there were claims by different authors, that the positive results were only due to a "changed psychological situation" of the treated patients. Which of these three theories is correct can not be decided yet. Whether blood purification methods have anything to offer for the treatment of some types of schizophrenic diseases requires further research.