The Iowa caucus has been the kickoff event for the Presidential nominating race since 1972. On the Democratic side, 7 candidates have won the caucus and gone on to win the nomination. These are Jimmy Carter in 1980, Walter Mondale in 1984, Bill Clinton in 1996, Al Gore in 2000, John Kerry in 2004, and Barack Obama in 2008 and 2012.
In the Republican party, 6 candidates have won the Iowa caucus as well as the nomination for President. These are Gerald Ford in 1976, Ronald Reagan in 1984, George H.W. Bush in 1992, Bob Dole in 1996, and George W. Bush in 2000 and 2004.
However, a number of these candidates were incumbent Presidents, such as Carter in 1980, Clinton in 1996, and Reagan in 1984. The track record is less good for candidates in years when their party did not already hold the Presidency.