Decision-making capacity (DMC) is an indispensable prerequisite for medical treatment choices, including consent to treatment, treatment discontinuation, and refusal of treatment. In patients with dementia, DMC is often affected. A particular challenge in assessing DMC are cognitive fluctuations that may lead to a fluctuation in DMC as well. Cognitive fluctuations are a diagnostic core feature of dementia with Lewy bodies and occur in Parkinson's and Alzheimer's diseases. In this article, these challenges are discussed and suggestions for assessing the DMC of patients with dementia with cognitive fluctuations are presented.