Kerry led the field in the vote, the first key test of Democrats vying to battle President George Bush in national elections in November, ahead of fellow Senator John Edwards.
Former Vermont governor Howard Dean, the longtime frontrunner in the Democratic race, trailed in third, the networks projected.
With 390 of 1993 precincts reporting, Kerry led with 36.6% of collected votes, ahead of Edwards with 34.3%, and Dean with 18%.
"Senator Kerry is doing very well and so is Senator Edwards. I congratulate them both, but we're determined to win," Dean told CNN television.
"If you told me a year ago I would finish third in Iowa, I would have been delighted. It's been a tough campaign. We've taken a lot of punches"
Democratic presidential candidate
"If you told me a year ago I would finish third in Iowa, I would have been delighted. It's been a tough campaign. We've taken a lot of punches," he said.
The favourites for the Iowa vote were Kerry, a senator from Massachusetts; Edwards, a senator from North Carolina; Howard Dean; and Dick Gephardt, a congressional representative from Missouri.
A victory in Iowa gives a candidate an ideal start as voters in each state pick their candidate to challenge President George Bush for the White House.
The states will later send delegates to the Democratic national convention in July to vote for presidential and vice presidential nominees.
The Iowa results – the first in a seven-month marathon of state-by-state elections - are not hugely significant by themselves, say political analysts.
But it can give a contender some useful early momentum when taken together with the New Hampshire primary.
"It can help candidates," Professor Stephen Hess of the Brookings Institution told Aljazeera.net on Monday.
"Kerry may do well in New Hampshire if he does well in Iowa."
Gephardt may pull out of the
race after the Iowa result
But the Iowa caucus election was most crucial for Gephardt, a Midwestern Democrat grandee who has invested heavily in the state.
"It's a must-win for Gephardt," Hess told Aljazeera.net. "He's from neighbouring Missouri, he won in Iowa in 1998, and he has put all his chips in this state."
Gephardt's intense campaigning and emphasis on issues dear to Iowans – jobs and trade – means if he cannot win here, the former House majority leader may well drop out of the race.
On the other hand, said Hess, the pressure is off Edwards because he was not initially expected to do well.
Other candidates include the conservative Connecticut Senator Joe Lieberman, who has reserved his campaigning efforts for New Hampshire; leftwing Ohio Congressman Dennis Kucinich; and black civil rights activist Al Sharpton.