The new EU regulation on clinical trials is intended to promote a greater level of harmonization of European Union rules in this area. However, it does not elaborate a common normative framework regarding the functioning of research ethics committees, leaving this responsibility to the Member States. This article offers a comparative analysis of the resulting regulatory situation. It demonstrates that this scenario is defined by considerable variability in the regulation of ethics monitoring between the EU Member States. We argue that this disparity should not necessarily be a negative factor for the optimization of the trial supervision regime in the EU. Moreover, we consider that it may be a stimulus for the achievement of excellence in the performance of this monitoring task. On the other hand, we also highlight risks for the rights of participants if an adequate monitoring framework is not ensured. Under these circumstances, we observe how the EU faces a dilemma. On the one hand, it may promote a rigid uniformity between the regulation of ethics committees between Member States, but this might diminish the quality of their performance. On the other hand, it may opt for maintaining the current situation, but this might increase differences in the performance of the ethics committees between Member States, including the number trials performed by country. A third option would be to allow the competitive framework to remain for a set period of time, in order to learn from the best practices reached in individual Member States before finally harmonizing national legislative provisions on this basis.This work was supported by Eusko Jaurlaritza [grant number Ayudas a grupos de investigación IT-1066-16]; H2020 Science with and for Society [grant number GRANT AGREEMENT NUMBER — 788039 — PANELFIT]