Studies of international regimes, law, and negotiation, as well as regional integration, near universally conclude that political entrepreneurship by high officials of international organizations—"supranational entrepreneurship"— decisively influences the outcomes of multilateral negotiations. Studies of the European Community (EC) have long stressed their informal agenda-setting, mediation, and mobilization. Yet the studies underlying this interdisciplinary consensus tend to be anecdotal, atheoretical, and uncontrolled. The study reported here derives and tests explicit hypotheses from general theories of political entrepreneurship and tests them across multiple cases (the five most important EC negotiations) while controlling for the actions of national governments. Two findings emerge: First, supranational entrepreneurship is generally redundant or futile; governments can almost always efficiently act as their own entrepreneurs. Second, rare cases of entrepreneurial success arise not when officials intervene to help overcome interstate collective action problems, as current theories presume, but when they help overcome domestic(or transnational) collective action problems. This suggests fundamental refinements in the core assumptions about transaction costs underlying general theories of international regimes, law, and negotiation.