Some elderly members of a small community in the state of Rio Grande do Sul, Brazil were victims of fraudulent behavior by a group of criminals. It happened shortly before the development of a research project that evaluated quality of life and decision-making capacity. Such a crime was a significant trauma for the victims and, as a result, the participants of the research refused to sign the Informed Consent Form. This case serves as background for supporting the distinction – sometimes confused in the literature - of emerging issues in the field of bioethics: ´autonomy` and ´self -determination`. Ironically, although they had autonomy to decide to take part in the research, they were in fact not fully prepared to exercise their self-determination, since they were not able to sign a document to support their choice. Also, draws attention to the difficulties of obtaining scientific data from vulnerable populations.