At least seven different sets of criteria are commonly used for the diagnosis of vascular dementia (VaD). These are the ischemic scales (IS) of Hachinski, Rosen and Loeb, the criteria from the DSM-III-R, those outlined by Erkinjuntti et al., the State of California Alzheimer''s Disease Diagnostic and Treatment Centers (ADDTC) and the international workgroup of the American National Institute of Neurological Disorders and Stroke (NINDS) and the European ''Association Internationale pour la Recherche et l''Enseignement en Neurosciences''. To investigate the differences and similarities of these criteria, we applied them to a sample of 124 demented patients from the Maastricht Memory Clinic. Only 8 patients were diagnosed as having VaD by all criteria. Depending on which criteria were used, the frequencies of VaD and Alzheimer''s disease (AD) ranged from 6 to 32%, and from 48 to 56%, respectively. The IS of Hachinski and Rosen resulted in the highest frequencies of VaD, whereas the criteria of Erkinjuntti and those from the ADDTC and the NINDS workgroup yielded the lowest. The number of patients with VaD was reduced substantially when neuroradiological data and the temporal relationship between stroke and dementia were taken into consideration. We conclude that the seven sets of criteria cannot be regarded as interchangeable. Differences in the criteria for VaD and AD may be an overlooked source of interstudy variance.