Evaluation of changes in functional performance and activities of daily living skills is an essential aspect of the assessment of elderly individuals with chronic illness. Although functional decrement is a central aspect of Alzheimer's disease (AD), many measures currently utilized to assess these changes have limitations. Empirical and systematic examination of the functional changes occurring in patients with AD has resulted in the development of an assessment measure termed Functional Assessment Staging (FAST) that allows for the specific evaluation of these changes throughout the entire course of AD. In this paper the results of three separate investigations regarding the reliability, validity, and progressive ordinality of FAST are described. The results indicate that FAST is a reliable and valid assessment technique for evaluating functional deterioration in AD patients throughout the entire course of the illness. Moreover, the results suggest that the FAST elucidates a characteristic pattern of progressive, ordinal, and functional decline in AD. Because the elements of functional capacity incorporated in FAST are relatively universal and readily ascertainable, as well as characteristic of the course of AD, FAST can serve as a strong diagnostic and differential diagnostic aid for clinicians. The sensitivity of FAST to the entire course of AD, even in its most severe stages, may be indicative of the potential value of this instrument for further investigation of the temporal longitudinal course of AD, and of the relationships between clinical pathology and neuropathology throughout the entire longitudinal course of AD.