Subjects enrolled in studies testing high risk interventions for incurable or progressive brain diseases may be vulnerable to deficiencies in informed consent, such as the therapeutic misconception. However, the definition and measurement of the therapeutic misconception is a subject of continuing debate. Our qualitative pilot study of persons enrolled in a phase I trial of gene transfer for Parkinson disease suggests potential avenues for both measuring and preventing the therapeutic misconception. Building on earlier literature on the topic, we developed and (...) tested an interview guide that focuses on how the subjects decided to participate, emphasizing the integration of subjects’ various statements that are relevant to assessing the therapeutic misconception, rather than evaluating them as isolated statements. The results indicate that a subject’s understanding of the purpose of research is best explored in juxtaposition to the subject’s motivation for participating. (shrink)