To determine if Alzheimer’s disease (AD), its Lewy body (LB) variant (LBV), and diffuse LB disease (DLBD) are distinguishable at initial clinical evaluation, data from autopsy-confirmed AD, LBV, and DLBD were examined. No significant differences were found in age at onset, age at death, total duration of illness, duration of illness before initial visit, duration of illness from initial visit to death, or severity of illness at initial evaluation. Hallucinations and delusions were significantly more frequent for LBV and DLBD, respectively, than for AD, and falls were more frequent for DLBD than for AD. Extrapyramidal symptoms (EPS) were less frequent in neuroleptic-free AD subjects than in LB subjects; the percentage of AD patients with EPS after neuroleptic exposure was less than that among LB patients. Seizures were significantly more common for DLBD than for AD or LBV. LB dementias differed from AD at initial evaluation, with more frequent hallucinations and delusions, EPSs, and seizures, and longitudinally in neuroleptic sensitivity, but the data did not distinguish LBV from DLBD.