The letter of the Muscovy ruler Boris Godunov which is held in the Library of Kórnik, is the earliest historical document that testifies about the relations between the government of Muscovy and Ukrainian Cossacks since the end of the sixteenth century.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 is reviewed the historical context of such contacts and their long term consequences. At that time, led by Krzysztof Kosynskyi, Cossacks entered into conflict with the powers of the Commonwealth. Godunov...