In recent years, the topic of differentiated integration in the European Union has become increasingly discussed in both political science research and politics in general. Whereas differentiated integration is viewed as necessary for deeper cooperation, recent findings suggest that it increases the gulf between participants and non-participants, making it difficult for non-participating countries to join in later negotiations. However, there is a lack of theoretical and empirical work regarding the relationship between different levels of participation in the EU and national policy outcomes. This article addresses this question by comparing the policy outcomes in fully participating, selectively participating (opting-in) and non-participating (opting-out) EU Member States relative to EU legislation. The findings show that selective participation (opting-in) increases state conformity with EU laws relative to no integration at all (opting-out), but it does not completely bridge the gap between fully integrated Member States and non-participants. The results suggest that countries with flexible arrangements are generally less likely to implement EU laws than full participants, even when they choose to legally commit to the EU requirements. This finding raises some further questions about the rationale behind selective participation and its consequences for policy conformity, if its application expands to other policy areas and more Member States in the future.