A brief history and summary of studies designed to elucidate the role of monoamine oxidase (MAO) in schizophrenia are presented. The majority of these studies have reported a decrease in the platelet enzyme activity of chronic schizophrenic patients when compared to controls. Difficulties encountered when comparing MAO activity measured in different patient populations are also considered. Finally, the significance of decreased platelet MAO activity is discussed with respect to its possible etiological role in some forms of schizophrenia.