For nearly three decades, prevention, and especially the indicated prevention of severe mental disorders has increasingly become a target of psychiatric research and, consequently, of ethical debates. The focus and the criticism of the latter are on the accuracy of prediction and the safety of preventive interventions, though concerns and arguments vary with the different characteristics of the considered disorder. Taking endogenous psychoses, Alzheimer’s dementia and Huntington’s disease as examples, costs and benefits of an early risk assessment, of an early preventive intervention and of prevention research will be examined with special emphasis on aspects of patient’s autonomy including considerations on informed consent and the ethical principles of nonmaleficence and beneficence.