What ethical justification can be found for informing a person that he or she will later develop a lethal disease for which no therapy is available? This question has been discussed during the past twenty years by specialists concerned with the prevention of Huntington's Disease, an incurable late-onset hereditary disorder. Many of them have played an active role in developing experimental testing programmes for at-risk persons. This paper is based on a corpus of 119 articles; it reviews the development of their reflection and includes an outline of the ethical problems identified and the solutions adopted in pre-clinical protocols. Seen in a broader perspective, the experience of presymptomatic testing for Huntington's Disease has given medical geneticists the opportunity to clarify their ethical position in the as yet little explored field of predictive medicine.