The letter of Muscovy ruler Boris Godunov which is held in the Library of Kórnik, is the earliest historical document that testifies about the relations between the government of Muscovy and Ukrainian Cossacks since the end of the sixteenth century. In the article are reviewed the historical context of such contacts and their long term consequences. At that time, led by Krzysztof Kosynskyi, Cossacks entered into conflict with powers of Commonwealth. Godunov informed Cossacks that his government invites them for military service. It was the first time when Cossacks obtained the opportunity to make a choice between the service to the Polish King or Muscovy Tzar. During the next a half of the century, Ukrainian Cossacks transformed into a powerful military corporation, which influenced on the balance of powers in international relations of the region. Later on, when in the consequence of the war with Poland, Cossacks switched themselves to the side of Muscovy in 1654, this change provided a foundation for the hegemony of Muscovy in East Europe.