A diagnosis of Alzheimer's disease (AD) is usually made after excluding more or less obvious other causes of dementia. The diagnostic criteria for AD of DSM-III-R and McKhann et al. (1984) are based upon such an approach. Recently two disorders have been described; diffuse Lewy body disease (DLBD) and forntal lobe dementia (FLD), which may resemble AD to such an extent that diagnostic criteria for AD are fulfilled. In this papier studies on the clinical picture and the neuropathological correlate of DLBD and FLD are reviewed. Implications for patient care and for research of these two `real' differential diagnostic possibilities for AD are discussed. Finally, some guidelines for differentiating AD, DLBD and FLD are given.