Introduction The attitude of medical students towards living kidney donation (LKD) is of great interest given that they will become promoters of this technique in the near future. Objective To analyse the attitude of Spanish medical students towards related and unrelated LKD and to determine the factors affecting this attitude. Materials and methods Type of study: A sociological, interdisciplinary, multicentre, and observational study. Study population: Medical students enrolled in Spain (n = 34.000). Sample size: A sample of 9598 students (99 % confidence and precision of ±1 %), stratified by geographical area and academic year. Measurement instru ment: A validated questionnaire (PCID-DVR RIOS) was administered and completed anonymously. Results There was a completion rate of 95.7 % (n = 9275); 93 % (n = 8630) were in favour of related LKD, and 30 % (n = 2784) were in favour of unrelated LKD. The following factors were associated with this attitude: (1) age (p = 0.008); (2) sex (p < 0.001); (3) year of university degree (p < 0.001); (4) a belief that a transplant might be necessary in the future (p < 0.001); (5) attitude towards deceased organ donation (p < 0.001); (6) a willingness to accept a...