Introduction : The risk of colorectal cancer (CRC) is increased in first-degree relatives of patients with CRC or advanced adenoma, for whom targeted screening by colonoscopy is recommended. The rate of completion of this examination is insufficient. The main hypothesis is that patients are not sufficiently aware of the familial implications of their disease or have difficulty informing their relatives. This study is part of the development of a personalized prevention education (PE) program based on sessions inspired by motivational interviewing. The aim of index-case education was to encourage participants to engage with their relatives to promote CRC screening among them. Aim of the study : This ancillary study, which complemented a cluster-randomized trial, aimed to qualitatively measure the performance of this PE. Methods : Based on a qualitative methodology, semi-structured interviews were conducted with14 index-case patients and three trainers (FTs) who delivered PE to them. A thematic content analysis followed by a textual analysis, and then a cross-analysis of these two methods, were performed. Results : The patients’ discourse is shaped by their experience of the disease, family relationships, and interactions with healthcare providers. The linear biographical approach allows them to “re-situate” the PE, but few precise methodological elements are reported. Nevertheless, both analyses highlight a predominantly positive experience of the PE. A majority of the trainers’ discourse focuses on the reception of the patients’ accounts of their experiences, and the methodology is unclear. The trainers report that the training and PE sessions are too short and that there is a lack of clarity about their role and stance, preventing them from fulfilling the objective. Conclusions : Despite patients’ generally positive experience of the PE, this study did not clearly establish its relevance and methodological coherence. While not fundamentally called into question, this methodology likely requires revision and improvement.