Clinico-pathological studies have shown that the clinical diagnosis of multi-infarct dementia (MID) is even more difficult than that of dementia of the Alzheimer type (DAT). The study evaluated the significance of course characteristics for the diagnosis of MID and DAT. Course characteristics were rated when 57 demented patients were admitted to our neurogeriatric department. Diagnosis of MID and DAT, respectively, was established after a follow-up study with repeated neurological, psychiatric and neuropsychological investigations. In 21 cases diagnosis was confirmed by postmortem neuropathology. MID lacked the typical course of the disease in about two thirds of patients, while most DAT patients presented with the typical course of primary degenerative dementia. Features of the "typical" clinical course of MID (abrupt onset, stepwise deterioration) helped to exclude DAT, whereas MID could not be excluded on the basis of a history of insidious onset and gradual decline.