This article reviews selected recent findings which contribute to the delineation of the early onset, extrapyramidal symptom, and Lewy body subtypes. These examples underscore certain potential advances and pitfalls in research which attempts to define subtypes. A subtype of Alzheimer's disease is defined by the presence of a specific etiology, a set of clinical signs, a prognosis, and a response to treatment. Cognitive decline can also occur on the basis of factors not intrinsic to a particular subtype. The term ‘excess disability’ refers to a patient's functioning which declines more rapidly than is expected on the basis of cerebral pathology alone. This article discusses some relevant studies which show how the concept of ‘excess disability’ contributes to understanding the total disability of the Alzheimer's patient.