For nearly two 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 main focus and critic of the latter is on the accuracy of prediction and the safety of treatment, though concerns and arguments vary with the different characteristics of the considered disorder. Taking endogenous psychoses, Alzheimer’s disease (AD) and Huntington’s disease (HD) as examples, costs and benefits of an early detection, of an early intervention and of prevention research in the prodromal or premorbid phase, though heavily intertwined, 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.