* Buffett, Gates ask rich to give away wealth to charity

* Giving Pledge campaign targeting U.S. billionaires (Adds comment from Eli and Edythe Broad)

NEW YORK, June 16 (Reuters) - Investor Warren Buffett and Microsoft MSFT.O founder Bill Gates announced on Wednesday that they are asking hundreds of billionaire Americans to give away at least 50 percent of their wealth to charity.

Buffett, who made his fortune with insurance and investment company Berkshire Hathaway Inc BRKa.NBRKb.N, Gates and his wife, Melinda, have held a series of dinners with a couple dozen rich Americans in the past year to urge them to make a philanthropic pledge.

They have named the campaign the Giving Pledge and are asking those who commit to giving away at least half their fortune during their lifetime or after their death to publicly state their intention with a letter explaining their decision.

Patty Stonesifer, a former chief executive of the Gates Foundation and now adviser to Gates and Buffett, said that four families had agreed to announce their pledge on Wednesday -- real estate and construction billionaire Eli Broad, venture capitalist John Doerr, media entrepreneur Gerry Lenfest and former Cisco Systems CSCO.O Chairman John Morgridge.

Broad and his wife Edythe said in a statement that they will pledge to give away 75 percent of their wealth during and after their lifetime. Forbes has estimated Broad to be worth $5.7 billion.

“We agree with Andrew Carnegie’s wisdom that ‘The man who dies rich, dies disgraced,’ and we also believe ‘he who gives while he lives also knows where it goes,’” the couple said. “Philanthropy is unbelievably rewarding.”

Lenfest has already given away more than $800 million, or about 65 percent of his fortune, a spokeswoman said. The amount pledged by Doerr and Morgridge was not immediately available.

In a letter posted on the campaign’s website, www.givingpledge.org, Buffett said he “couldn’t be happier” with his 2006 decision to give away 99 percent of his wealth to the Bill & Melinda Gates Foundation and family charities.

“Now, Bill and Melinda Gates and I are asking hundreds of rich Americans to pledge at least 50 percent of their wealth to charity,” Buffett, 79, said.

He said that so far about 20 percent of his Berkshire Hathaway shares, including shares given by his late wife Susan, had been distributed to charity and that he would continue to distribute annually 4 percent of the shares he retains.

SPENDING ON CURRENT NEEDS

“At the latest, the proceeds from all of my Berkshire shares will be expended for philanthropic purposes by 10 years after my estate is settled. Nothing will go to endowments; I want the money spent on current needs,” Buffett wrote.

Gates was ranked the second richest man in the world by Forbes magazine this year with $53 billion, while Buffett came in at No. 3 with $47 billion. Forbes said the United States is home to 403 billionaires, the most in the world.

Bill and Melinda Gates have so far given more than $28 billion to their foundation. Since the foundation began in 1994 it has given away more than $22 billion for health improvements in poor countries and to improve access for Americans to opportunities they need to succeed in school and life.

The Giving Pledge will not accept any money; it is simply asking billionaires to make a moral commitment to give away their wealth to charity.

While the campaign is targeting billionaires, the Giving Pledge said it is “inspired by the example set by millions of Americans who give generously (and often at great personal sacrifice) to make the world a better place.” (Editing by Eric Beech and Cynthia Osterman)