The Clinical Dementia Rating (CDR) scale is broadly accepted by clinicians as a staging measure for Alzheimer's disease (AD). Extensive assessment of its validity is, however, lacking. We examined the concurrent and predictive validity of both summary CDR scores (global CDR, Sum of Boxes [sum of scores on the individual components]) and scores on two (Memory and Orientation) of the CDR's six components (the other four components are judgement and problem-solving, community affairs, home and hobbies, personal care) using cross-sectional, longitudinal and survival information on 434 community-resident probable AD patients. Cross-sectionally the Orientation box score correlated substantially with an independent neuropsychology measure of orientation, but the Memory box score correlated more poorly with an independent measure of memory than with any other neuropsychology measure. The relationship of the global CDR score and the Sum of Boxes score to scores on neuropsychology measures was comparable to that of the Orientation and Memory box scores. Longitudinally, Memory box score a year later was predicted equally well by the other box scores (personal care excepted). The individual components were comparable to both summary CDR scores in predicting time to death. The CDR has content and criterion validity. However, since intentionally all components measure aspects of cognitive functioning, they are closely related. Nevertheless, sufficient distinctions remain that assessment in each area is still warranted.