Extending a dataset compiled by Sweet and Brunell (1998b), we investigate the factors influencing the choice of domestic judges to pass on cases to the Court of Justice of the European Union. While EU judicial scholars have typically relied on integration theory or ad hoc theories of adjudication to explain the conduct of judicial actors, we present an account of judicial decision making in the context of the preliminary ruling mechanism more consonant with recent theoretical advances in judicial behaviour research. Based on a Bayesian framework, our estimation strategy addresses issues of endogeneity and distributional assumptions neglected in the previous literature. We find that the rate at which domestic courts refer cases is influenced by litigation volume, familiarity with EU law and political fragmentation, but neither by monism, nor by intra-EU trade. The paper also provides the first full-scale analysis of the impact of enlargement on preliminary references.