The loser of Lithuania's 2003 presidential election has won Sunday's new vote, called after the impeachment of his rival, preliminary results say.
Adamkas is a former US citizen who lost to Mr Paksas in 2002
But Valdas Adamkus is set for a run-off with former Prime Minister Kazimeira Prunskiene as he failed to get the 50% of the vote needed for an outright win.
He had 30% to 22% for Ms Prunskiene, the election commission said.
The 2003 winner, Rolandas Paksas, was barred from standing after being sacked on corruption charges he denies.
The second round will be held on 27 June.
Lithuania on Sunday also voted in the European elections for the first time.
Some 2.6 million eligible voters in the country - one of the 10 new European Union members - were electing 13 MEPs in the EU parliament.
Mr Adamkus, a 77-year-old former US citizen, was Lithuanian president up to his shock defeat by Mr Paksas and led the other four contenders in numerous opinion polls.
Ms Prunskiene, 66, was Lithuania's first prime minister after the former Soviet republic declared independence in the early 1990s.
After Mr Paksas's impeachment, Arturas Paulauskas was appointed acting president.
Mr Paksas was impeached for allegedly leaking classified material and giving a Russian businessman citizenship for financial support - charges he fervently denies.
Paksas still enjoys popular support in the Baltic state
The flamboyant 47-year-old former stunt pilot insists he is a victim of a political vendetta and says he did nothing wrong.
Correspondents note that Mr Paksas still enjoys popular support, mainly among those who have missed out on economic growth in the former Soviet state, which became an EU member on 1 May.
Earlier this year, Lithuania also joined Nato.