Loss of capacity to consent to medical treatment (hereafter competency) is an inevitable consequence of Alzheimer's disease (AD) and has important legal and ethical implications. However, the medical-legal construct of competency remains poorly understood, particularly in dementia and aging populations. We investigated the construct through a series of exploratory and validation factor analyses of performance by 82 patients with probable or possible AD on the Capacity to Consent to Treatment Instrument (CCTI). In the exploratory phase, principal components analyses revealed that the CCTI is composed of two orthogonal factors: Verbal Conceptualization/Reasoning and Verbal Memory. In the validation phase, principal components analysis of individual factor scores and neuropsychological test performance supported and further elaborated the two factor structure. The findings suggested that consent capacity in an AD population is a multidimensional construct represented by neurocognitive factors of verbal reasoning and verbal memory. Measures of verbal conceptualization/reasoning and verbal memory are likely to be sensitive to declining treatment consent capacity in older adults with AD. © 1999 Taylor & Francis Group, LLC. All rights reserved.