What does it mean to respect autonomy and encourage meaningful consent to treatment in the case of patients who have dementia or are otherwise incompetent? This question has been thrown into sharp relief by the Law Lords' decision in R.v Bournewood Community and Mental Health NHS Trust, ex parte L (1998). The effect of the Law Lords' ruling in the Bournewood judgment is to reinforce problematic and serious anomalies in the way we view patients whose competence is in doubt because of their mental disorder. Others, such as relatives and informal carers, are frequently allowed to decide on behalf of adults whose competence is doubtful in a way that English law generally abhors, even for totally incompetent patients in a persistent vegetative state. This raises profound questions about autonomy. And incompetent adults' consent to treatment is not required to be of the same quality as it is for the rest of us: mere absence of resistance will do. This paper will explore the philosophical, jurisprudential and legal implications of this difference. Throughout I will be more concerned with the ramifications of a finding of incapacity than with how such a finding is made (for the latter, see such classic texts as Applebaum & Roth (1982), Grisso & Applebaum (1998) and Bellhouse et al (2001)).