ENThe paper studies Polish-Lithuanian relations in the period of 1989-2004 with particular focus on the years 2002-2004 when the Treaty establishing the Constitution for Europe was being created. The first part of the paper analyzes Polish-Lithuanian relations in 1989-2002. It is a brief introduction to the subject with reference to the bilateral relations in the period of political transition in both countries, and the most significant documents signed at that time: consular convention, the "Declaration on Friendly Relations and Good-neighborly Cooperation between the Republic of Poland and the Republic of Lithuania" and the "Joint Declaration o f the Presidents of the Republic ofPoIand and the Republic of Lithuania" which marked a breakthrough in the foundation of good-neighborly relations between Poland and Lithuania. The second part focuses on Polish-Lithuanian relations in 2002-2004. In this period these relations were predominated by the fact that Lithuania was to join NATO and both countries were to join the European Union. Poland strongly supported Lithuanian efforts to become a member of the North-Atlantic Treaty, and assumed the role of her advocate in this matter. Over the period of two years both states collaborated in developing the most important statements in the Treaty establishing the Constitution for Europe and supported the introduction of the Invocation Dei and the mention of Christian values in the Treaty preamble. The second part of the paper characterizes also the cooperation of Polish and Lithuanian heads of parliamentary European committees concerning direct agricultural subsidies and the joint statement concerning cooperation in the efforts to join the European Union signed in Vilnius. The paper analyzes the results of the accession referendum in Poland and in Lithuania and the comments published in Lithuanian newspapers.Finally, the article analyzes Polish-Lithuanian relations after both countries joined the European Union, and it outlines the future perspectives of cooperation.