As persons with Alzheimer's disease (AD) lose their ability to make decisions, someone else has to make decisions for them. We performed a prospective cohort study of 77 AD patient-caregiver dyads to examine when this transition occurs. When dementia severity surpassed a threshold marked by a Mini-Mental State Examination (MMSE) score less than 20, the level of caregiver-reported patient involvement in the medical decision-making process declined (Moderate [MMSE = 19-12]: Odds Ratio [OR] = 2.35, 95% confidence interval [95% CI] = 1.01-5.49; P =.048; Severe [MMSE < 12]: OR = 29.38, 95% CI = 5.98-144.25, P <.001). Furthermore, older patients (OR = 1.06, 95% CI = 1.00-1.12, P =.049) and mounting caregiver burden (OR = 1.12, 95% CI = 1.04-1.26, P =.003) were significant independent predictors of transitions to the caregiver-dominated medical decision-making process. These results provide clinicians with prognostic information that can help caregivers understand how their role in decision making will change over the course of a patient's dementing illness.