ENThe article is devoted to the problem of studying interstate relations between the Grand Duchy of Lithuania and the Kingdom of Poland in the period from the conclusion of the Union of Kreva in 1385 to the Acts of Horodlo in 1413. The role and influence of the title of the highest prince of Lithuania (“supremus dus Lithuaniae”) as a means of implementation of the Jagiellonian idea and the control of the Polish king over the territory of the Grand Duchy of Lithuania. Keywords: Jagiellons, Union of Kreva, suzerain, Union of Vilnius-Radom, Acts of Horodlo.