BACKGROUND The aim of this study was to investigate the frequency and severity of neuropsychiatric symptoms (NPS) in patients with early onset Alzheimer's disease (EAOD) and late onset AD (LOAD). METHODS Patients were selected from a specialized memory outpatient clinic. The Mini-Mental State Examination, the Neuropsychiatric Inventory (NPI), and the Global Deterioration Scale results were analyzed. RESULTS By comparing EOAD (n = 35) and LOAD (n = 35) patients, no significant differences were found in clinical or demographic variables, matched for sex, education, and disease severity. There were no differences between groups in total NPI frequency or severity scores. The most common NPS were irritability, apathy, anxiety, and depression. We found an association of NPI scores with disease severity and duration, which was more specific in patients with LOAD and was also associated with the presence of delusions and hallucinations. CONCLUSIONS Despite subtle differences, NPS is considered important in the assessment of patients with AD, regardless of the age of onset.