OBJECTIVES: To better understand the nature of informed consent encounters for research involving patients with dementia that requires proxy consent. DESIGN: Audiotaping of informed-consent encounters for a study of genetic markers for sporadic Alzheimer's disease. SETTING: Outpatients at an Alzheimer's disease research center. PARTICIPANTS: Patients with dementia and their companions. MEASUREMENTS: Audiotapes were analyzed to characterize communication style and coverage of the standard elements of informed consent and, using the Roter Interaction Analysis System, to capture the dynamics of three-way interaction between the patient, their companion, and the physician investigator. RESULTS: Of 26 informed consent encounters, all involved a patient, a companion, and a physician. Patients had a mean Mini-Mental State Examination (MMSE) score of 21.8. For patients, 49% of their interactions involved agreement and approval (positive statements), 16% psychosocial information, 7% biomedical information, 7% asking questions, and 7% expressing emotion. Companion interactions involved 37% positive statements and 19% biomedical information. Physician interactions involved emotional expressiveness (30%) and positive statements (19%). Discussion length was positively related to MMSE score (Spearman rho=0.45; P<.02). Coverage of required elements of informed consent was fairly comprehensive and had no relationship to patients' MMSE scores. CONCLUSION: These data should inform policies regarding the ethically appropriate ways of conducting research with cognitively impaired adults. For example, patients in this study were more silent than their companions and the physician, but when patients spoke, they primarily agreed with what was said. Although this might first seem to signal assent, such an interpretation should be made with caution for persons with dementia. In addition, previous work on informed consent has focused on its cognitive aspects, but these data reveal that the emotional and social dimensions warrant attention.