Former Massachusetts Governor Mitt Romney has narrowly won the first vote by US Republican party supporters in the process to choose a presidential candidate for this year's election.
He finished just eight votes ahead of former senator Rick Santorum in the rural Midwest state of Iowa.
Ron Paul was third and Newt Gingrich fourth. Rick Perry came fifth and is considering the future of his campaign.
Michele Bachmann, who came sixth, is to hold a news conference later.
The Minnesota congresswoman is cancelling her campaign trip to South Carolina, her campaign manager said.
The Hawkeye state's caucuses were the first official contest to pick a contender to face Democratic President Barack Obama in the 6 November presidential election.
Mr Romney, appearing on national breakfast shows on Wednesday morning, said staying power set him apart from the runners-up.
"I've got a big target on me now," Romney said on breakfast television on Wednesday. "I've got broad shoulders. I'm willing to handle it."
Further caucuses and primary elections will be held in all 50 states, Washington DC and other territories over the next few months.
The eventual nominee will be formally anointed at the Republican National Convention in Florida this August.
Iowa is not usually a kingmaker - John McCain, the eventual Republican nominee in 2008, came fourth in the state's caucuses that year - but it will help shape the race for the White House.
Hours after the caucuses closed, Iowa party chairman Matt Strawn announced that Mr Romney had won by just eight of the 122,255 votes cast with 30,015 ballots against 30,007 for former Pennsylvania Senator Santorum.
Mr Santorum declared "Game on!" to cheering supporters as results continued to come in.
"You have taken the first step toward taking back this country," he said.
'No nasty ads'
Mr Paul, a Texas congressman, finished third with 21% of the vote and vowed to contest New Hampshire.
"This momentum is going to continue," he told a jubilant crowd of supporters. "We will go on, we will raise the money."
Finishing fifth, Texas Governor Rick Perry said he was returning to his home state in order to "determine whether there is a path forward for myself in this race".
Former House of Representatives Speaker Newt Gingrich, who saw a brief lead evaporate under a barrage of negative advertising last month, pledged to remain in the race and challenge Mr Romney, "a Massachusetts moderate".
"We are not going to go out and run nasty ads," said Mr Gingrich, who finished fourth.
"But I do reserve the right to tell the truth. And if the truth seems negative that may be more of a comment on his record than it is about politics."
Former Utah Governor Jon Huntsman was on the ballot, but did not campaign in Iowa.
In Iowa 122,255 voters turned out at more than 1,700 fire stations, schools, public buildings and private homes.
The campaign had been a roller-coaster since the summer, with Mrs Bachmann, Mr Perry and Mr Gingrich each taking a brief turn at the head of the pack.
Those three - along with ex-candidate Herman Cain - were buoyed by support from conservatives unsatisfied with Mr Romney, who held steady in second place in polling. But each in turn collapsed under further scrutiny or, in Mr Gingrich's case, heavy negative advertising.
Support for Mr Santorum grew fast in recent weeks.
"This is a Cinderella story," said Iowa Secretary of State Matt Schultz, who endorsed Mr Santorum. "Two weeks ago we were in last place."
A week after Iowa, the state of New Hampshire holds its primary election - Mitt Romney has a big lead and Mr Santorum consistently polls less than 5% there.
Republican Party officials say Senator McCain plans to endorse Mr Romney on Wednesday, a signal to the party faithful to rally around him.
Mr Santorum said that was to be expected. "John is a more moderate member of the Republican team, and I think he fits in with Mitt's view of the world," he said, AP reported.
Over the next six months, each US state will vote on the presidential contenders before a final nominee is selected.
Voters remain concerned by the slow pace of economic recovery from the recession that started during the end of the presidency of George W Bush and officially ended in 2009.