The President of the Bosnian Serb republic (Republika Srpska), Milan Jelic, has died of a heart attack, doctors and politicians say.
Mr Jelic's post was mainly ceremonial
"This is a sad day for Republika Srpska," the entity's prime minister, Milorad Dodik, told local TV.
Mr Jelic, 51, died after admission to hospital in Doboj on Sunday evening. He was president since October 2006 and headed Bosnia's football association.
The Republic Srpska shares Bosnia with the Muslim-Croat Federation.
The entities were set up under the Dayton accords which ended the 1992-95 war.
The country remains under international administration.
Mr Jelic, a businessman and economist, was a member of Mr Dodik's Social Democratic Party - the successor to the Communist Party.
While his post is mainly ceremonial, correspondents say he has backed Mr Dodik's opposition to calls for reforms by the EU, including efforts to unite the two entities' police forces.