In order to better understand students' adherence to the Mentoring Program at the School of Medicine, University of São Paulo (FMUSP), this qualitative study explored the reasons expressed by students for their involvement in the activity in 2004-2005, before and after structural changes in the program (inclusion of the mentoring program in the course hours). Students perform an annual assessment of the program through a structured questionnaire: "The Mentoree". This includes a specific question on attendance in the program, analyzed in this study. The findings show that exchange of experiences between students from different years, as mediated by a skillful mentor, promotes an expanded view of the course and the profession and motivates students to participate more in the program. Mentors that leave the program, irregular scheduling of meetings, ineffective communication, and discussion of uninteresting themes contribute to low adherence to the program. Inclusion of the program in the official course hours solved important problems, but many students still prefer to use the mentoring time for other academic activities or personal needs. Informal mentoring and groups with poor dynamics also explain limited participation (in the students' eyes). A mentoring program's success, as measured by students' participation, proves to be linked not only to an adequate structure, but also to the participants' personal characteristics and values.