Platelet monoamine oxidase activity levels have been evaluated in several neuropsychiatric disorders, to identify biochemical markers for pathological brain functioning. In the present work, we assayed both total and molecular monoamine oxidase activity in platelets of parkinsonian and demented patients: both showed significantly higher enzyme activity values than healthy controls. Thus, high platelet monoamine oxidase activity levels seem to be related to an increased intrinsic activity of single enzyme molecules. A significant positive correlation was found between platelet monoamine oxidase activity and severity of illness in both disorders: monoamine oxidase activity, therefore, may be considered as a state-dependent marker for neuro-degeneration. Such findings are discussed with reference to central nervous system biochemical abnormalities in parkinsonian and demented subjects; it might be that in both Parkinson's Disease and in dementia of Alzheimer type some central biochemical changes are reflected in certain peripheral tissues (thrombocytes, for instance), or that a systemic derangement accompanies the cerebral involvement.