The education debate in Spain has widely discussed the positive and negative features of grade retention on students’ academic performance but, in spite of its relevance, this impact has not been assessed beyond correlation. Because of that, in this research 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, approaching a causal influence. We find that repeating a grade in Spain seems to reduce students’ competences between 1.5 and 1.7 standard deviations. We conclude that an alternative to grade retention should be found, in order to prevent students from attaining lower their competences due to repetition.