The significance of clinically defined subgroups of Alzheimer's disease (AD) is discussed within a framework of etiologic, pathogenetic, and treatment issues. Cerebral amyloid angiopathy and familial AD are proposed as etiologic subtypes. Clinical heterogeneity associated with age at onset and sex are thought to reflect the presence of secondary pathogenetic factors. Longitudinal studies are necessary to clarify the meaning of various symptomatic patterns.