This work covers Czech-US relations starting in the 1990''s till the Eastern enlargement of the European Union in May 2004. It focuses primarily on the change in the mutual relations of the two countries after this historic enlargement. The paper is covering mainly political and economic dimensions of the relations. First, an overview of the formal background for the political relations is provided, followed by hypothesis regarding possible changes in relation to the new limitations coming with EU membership. To confront the formal basis with the real issues in the mutual relations the diplomatic matters in the past fifteen years are shortly covered. Next, the economic angle of the relations presents the character of the trade relations between the two states and its development over the aforementioned decade. Further, the potential changes in the Czech- US economic relations caused by the enlargement are outlined including opinions and expectations of experts. The paper concludes with summary of the observed and further expected changes in the Czech-US relations related to the EU entry.