The number of interest organizations (density) varies across policy domains, political issues and economic sectors. This shapes the nature and outcomes of interest representation. In this contribution, we explain the density of interest organizations per economic sector in the European Union on the basis of political and economic institutional factors. Focusing on business interest representation, we show that economic institutions structure the 'supply' of interest organizations by affecting the number of potential constituents, the resources available for lobbying and the geographical level of collective action of businesses. In contrast, we do not find consistent evidence that political institutions produce 'demand' for interest organizations by making laws, developing public policy or spending money. This is in contrast to the extensive evidence that such factors affect lobbying practices. The European Union interest system is (partially) shaped by economic factors, relatively independent from public policy or institutions.