Despite sensitization of medical students toward the donation of organs, a non-negligible percentage of students are not in favor of donation. To analyze the reasons of Spanish medical students who do not have a favorable attitude toward the donation of their own organs after death. The population under study is medical students in Spanish universities, using the database of the International Donor Collaborative Project, stratified by geographic area and academic year. The questionnaire completion was anonymous and self-administered. The study group is medical students with an unfavorable attitude toward organ donation. The assessment instrument used is a validated questionnaire of attitude toward the donation of organs for transplant, “PCID-DTO-Ríos.” The reasons against the donation are valued in the questionnaire through a question. Of the participants included in the PCID, students who are not in favor of organ donation were selected (n = 1899). Of them, 8.1% (n = 154) are against and 91.1% (n = 1745) are doubtful. The main reasons indicated are the fear of apparent death in 11.4% of respondents, fear of possible mutilation after donation in 11.1%, and religious reasons in 2.6%. Of those, 6.9% indicate other reasons but do not clearly specify the reasons, using words such as “fear” (2.5%) or “doubts about the process” (4.1%); 66.2% (n = 1257) indicated an “assertive refusal” (“I don't want to express my reasons”). Twenty percent of Spanish medical students are not in favor of donating their organs and are not in favor of showing their reasons. •Most Spanish medical students who are not in favor of organ donation do not indicate their reasons.•The main reason among Spanish medical students for not being in favor is the fear of apparent death.•Spanish medical students who are not in favor of organ donation are mainly in the first years of their career and without prior awareness of the subject.