Post mortem brain studies indicate that alterations in serotonergic and catecholaminergic systems might be associated with Alzheimer's disease (AD). The aim of the study was to determine serotonin (5-HT) levels and monoamine oxidase type B (MAO-B) activity in platelets of psychotic and non-psychotic patients with AD, established according to the NINCDS-ADRDA and DSM-IV-TR criteria. Cognitive impairment and psychotic features were evaluated using Mini Mental Status Examination and Neuropsychiatric Inventory. Platelet 5-HT concentration and MAO-B activity were determined spectrofluorimetrically in 116 (51 male, 65 female) healthy subjects and 70 psychotic (10 male, 60 female) and 151 non-psychotic (32 male, 119 female) patients. Psychotic and non-psychotic female and psychotic male patients had significantly lower platelet 5-HT concentration than corresponding sex matched control subjects. Platelet MAO-B activity was significantly increased in both male and female non-psychotic patients compared to the sex matched controls. Non-psychotic female patients had significantly higher platelet MAO-B activity than psychotic female patients. Our data suggest that platelet MAO-B activity, but not platelet 5-HT concentration, could differentiate between psychotic and non-psychotic subtypes of AD.