No consent capacity study has examined this in the context of an actual AD clinical trial. We assessed the capacity of persons with AD to provide informed consent to enroll in a randomized and controlled trial (RCT) by measuring their abilities to understand, appreciate and reason; and the cognitive and functional correlates and significance of their performance on these measures. Research coordinators at 13 Alzheimers Disease Cooperative Study (ADCS) study sites used the MacAthur Competency Assessment Tool for Clinical Research (MacCAT–CR) to measure the capacity of 59 subjects with very mild to moderate AD (MMSE 20.8±4.8, range 8–28) enrolled in the ADCS RCT of simvastatin. Pearson correlations compared performance on capacity measures with MMSE, ADAS–cog, and financial capacity. Three psychiatrists without access to patient cognitive data reviewed independently the MacCAT–CR interviews to judge whether a subject was capable of consenting. The agreement of at least two experts defined a subject as capable to provide informed consent. Subjects showed substantial variability on the measures of understanding (17.3±6.6), appreciation (3.4±2.0), and reasoning (5.8±1.8). Performance was correlated with overall cognition as measured using the MMSE (0.61, 0.38, and 0.29; all p 0.03) and ADAS–cog (–0.7, –0.51, –0.46; all p 0.0002), and with performance on a measure of financial capacity (0.75, 0.54, 0.48; p 0.0002). There was good agreement among the expert raters on their capacity judgments (kappa=0.73, p<0.0001). They found 52% (31/59) of the subjects capable of providing informed consent. Most (77%) capable subjects had very mild or mild AD as defined by MMSE>19. In contrast, not capable subjects were split between those with very mild or mild (46%) or moderate (54%) AD. Good correlations with measures of overall cognition and financial capacity and good inter–rater agreement support that the MacCAT–CR provides useful data on the capacity of persons with AD and is feasible in an actual AD RCT. The relationships between expert judgments and clinically relevant cutpoints on MMSE suggest that eligibility criteria can be set to assure a high likelihood that subjects are capable of informed consent in the event a trial needs to restrict enrollment to these persons.