Grade retention is at the core of the education debate in Spain, to the extent that its impact on students' competences has not been assessed beyond correlation. Because of that, in the present study, we analyse the influence of grade retention on students' competences, using more than 146,000 students from 6 PISA cycles (2003-2018) and an instrumental variable approach, in order to approach a causal influence. Our results show that repeating a grade in Spain seems to reduce students' competences between 1.5 and 1.7 standard deviations. Based on these results, we conclude that the Spanish educational authorities should find an alternative to grade retention, in order to prevent students from attaining a lower competence level due to repetition.