To examine the relationships between Alzheimer's disease (AD) severity and patient participation in decisions about their medical care, we performed a cross-sectional study of 74 caregivers' ratings of the degree to which AD patients participate in making decisions about their medical care. The majority of the caregivers reported that they made the final decisions about medical care (n = 48, 64%), one-quarter (n = 19, 26%) reported an equal role, and a small proportion (n = 7, 9%) reported that the patient made the final decisions. Multivariate models showed that the significant predictors of the patient's participation in decisions about medical care were measures of dementia severity: Mini-Mental State Examination (MMSE) and Clinical Dementia Rating (CDR). Patients with mild-stage disease (CDR = 1 or MMSE > or = 20) are involved in decision making largely in a collaborative role with the caregiver. Beyond mild-stage disease, the clinician can largely expect that the caregiver is making the final decisions. These results illustrate how and when caregivers can expect changes in their decision making role.