Platelet monoamine oxidase B (MAO-B) activity has been found to increase significantly in demented patients. For the first time, a 4-year follow-up of platelet MAO-B activity and Mini-Mental State (MMS) was performed in patients with probable dementia of the Alzheimer type (DAT) and age-matched controls. MAO-B activity of DAT patients increased significantly 2 years after the beginning of the study as compared with controls and remained significantly higher for the entire period of the examinations (p < 0.0001). The decrease of the MMS scores did not correlate with the time course of the increase of platelet MAO-B activity (Spearman rank correlation test). The decline of the MMS scores of DAT patients preceded the elevation of MAO-B activity. Since degenerative processes in brain areas which are responsible for cognitive function and are reflected by the MMS scores rather affect cerebral cholinergic than monoaminergic neurotransmitter systems, degeneration of the latter at late stages of DAT might be reflected by increased platelet MAO-B activity.