Bolesław was the eldest son of the Polish high duke Henry II the Pious by his wife Anna, a daughter of the Přemyslid king Ottokar I of Bohemia.
There was a similar situation in Greater Poland: after hearing the news of the defeat of Henry II in Legnica, Duke Przemysł I and his brother Bolesław the Pious retook the estates of Kalisz which once had been ruled by their father, the late Duke Władysław Odonic.