Despite the introduction of formal clinical criteria for Alzheimer disease (AD), the clinical diagnosis of AD remains one of exclusion of other dementias. To determine the accuracy of a clinical diagnosis of AD as made by practicing physicians, we reviewed the clinicopathologic records of a dementia brain bank and summarized the literature. Of 650 demented patients diagnosed during life as having AD, at autopsy 505 (78%) had AD with or without other neuropathologic conditions; only 390 (60%) of these had AD as the only neuropathologic condition. Of the remaining 145 (22%) patients with no neuropathologic evidence of AD, 39 had the nigrostriatal changes of Parkinson disease (PD), 25 had nonspecific degenerations, 15 had Pick disease, 14 had multiple infarcts, and 11 lacked any neuropathologic abnormality. Although the overall clinical accuracy for AD was lower than that summarized from the literature, clinical accuracy improved significantly between 1986 and 1990. In our broad sample of practitioners, accuracy of clinical diagnosis of AD may be improving, but continues to be hampered by difficulty in distinguishing the dementia of AD from certain dementing conditions and from AD mixed with other neuropathologic conditions.