A prospective series of consecutively admitted demented patients were examined in order to evaluate the role of common clinical methods in the differential diagnosis between Alzheimer's disease (AD) and vascular dementia. Patients fulfilling the DSM-III criteria for vascular dementia were divided into multi-infarct dementia (MID) and probable vascular dementia (PVD), the latter including also cases with combined vascular and degenerative dementia. The series consisted of 68 patients with AD, 79 with MID and 46 with PVD. These groups did not differ in regard to sex, age or degree of dementia. Absence of cardio- and cerebrovascular diseases differentiated AD from MID and PVD. Also absence of corticospinal tract signs and gait disorders differentiated AD from MID and PVD and that of bulbar signs AD from MID. Infarcts and white matter low attenuation on CT differentiated MID and PVD from AD. Altogether 88.6% of the MID patients and 41.3% with PVD had brain infarct on CT, but only one with AD. Ischemic scores seemed also to be useful in the differential diagnosis between AD and vascular dementia. Quantitative neuropsychology, EEG, routine cerebrospinal fluid and other laboratory investigations, including serum glucose and plasma lipids, seem to be less valuable.