The European countries have surpassed the United States with regard to comprehensive and stringent environmental regulation. This policy pattern has become very visible in the international debate about reduction of CO2 emissions. From a comparative public policy perspective, the recent difference is interesting because it restates the question about policy consequences of political institutions. This article addresses this theoretical question from the perspective of policy agenda-setting theory. We present the argument that while the U.S. political system, with its many “venues” that are relatively open to internal and external competing forces, might be quicker in generating attention to new issues like the environment, the institutionalization of attention is also less strong than in a European parliamentary system. The article substantiates this argument with empirics by comparing attention to and policymaking for environmental issues in the United States to Denmark.