A quarter of the European population lives in 'polycentric urban regions' (PURs): clusters of historically and administratively distinct but proximate and well-connected cities of relatively similar size. This paper explores whether tighter integration can increase agglomeration benefits at the PUR-level. We provide the first comprehensive list of European PURs (117 in total), establish their level of functional, institutional and cultural integration and measure whether this affects their performance. 'Performance' is defined as the extent to which urbanisation economies have developed, proxied by the presence of metropolitan functions. In this first-ever cross-sectional analysis of PURs we find that while there is evidence for all dimensions of integration having a positive effect, particularly functional integration has great significance. Regarding institutional integration, it appears that having some form of metropolitan co-operation is more important than its exact shape. Theoretically, our results substantiate the assumption that networks may substitute for proximity.