FORTUNE -- Just over a year ago, in May 2009, word leaked to the press that the two richest men in America, Bill Gates and Warren Buffett, had organized and presided over a confidential dinner meeting of billionaires in New York City. David Rockefeller was said to have been a host, Mayor Michael Bloomberg and Oprah Winfrey to have been among those attending, and philanthropy to have been the main subject.
Pushed by the press to explain, Buffett and Gates declined. But that certainly didn't dim the media's interest in reaching for descriptions of the meeting: The Chronicle of Philanthropy called it "unprecedented"; both ABC News and the Houston Chronicle went for "clandestine"; a New Yorkmagazine parody gleefully imagined George Soros to have been starstruck in the presence of Oprah. One radio broadcaster painted a dark picture: "Ladies and gentlemen, there's mischief afoot and it does not bode well for the rest of us." No, no, rebutted the former CEO of the Bill & Melinda Gates Foundation, Patty Stonesifer, who had been at the meeting and had reluctantly emerged to combat the rumors. The event, she told the Seattle Times, was simply a group of friends and colleagues "discussing ideas" about philanthropy.
And so it was. But that discussion -- to be fully described for the first time in this article -- has the potential to dramatically change the philanthropic behavior of Americans, inducing them to step up the amounts they give. With that dinner meeting, Gates and Buffett started what can be called the biggest fundraising drive in history. They'd welcome donors of any kind. But their direct target is billionaires, whom the two men wish to see greatly raise the amounts they give to charities, of any and all kinds. That wish was not mathematically framed at the time of the New York meeting. But as two other U.S. dinners were held (though not leaked), Buffett and Gates and his wife, Melinda, set the goal: They are driving to get the super-rich, starting with the Forbes list of the 400 wealthiest Americans, to pledge -- literally pledge -- at least 50% of their net worth to charity during their lifetimes or at death.
Without a doubt, that plan could create a colossal jump in the dollars going to philanthropy, though of what size is a puzzle we'll get to. To begin with, a word about this article you are reading. It is the first public disclosure of what Buffett and Melinda and Bill Gates are trying to do. Over the past couple of months Fortune has interviewed the three principals as the project has unfolded, as well as a group of billionaires who have signed up to add their names to the Gates/Buffett campaign.
In a sense this article is also an echo of two other Fortune stories, both featuring Buffett on the cover. The first, published in 1986, was "Should you leave it all to the children?" To that query, Buffett emphatically said no. The second article, "Warren Buffett gives it away," which appeared in 2006, disclosed Buffett's intention to gradually give away his Berkshire Hathaway (BRK.A) fortune to five foundations, chief among them the world's largest, the Bill & Melinda Gates Foundation. (For Buffett's thinking on the disposition of his wealth, see "My philanthropic pledge.")
Since then, in four years of contributions, Buffett has given the foundation $6.4 billion, not counting the 2010 gift, to be made this summer. The foundation in turn has in that same period combined Buffett's money and its immense gifts from the Gateses to raise its level of giving to about $3 billion a year, much of it for world health. One small example: the Medicines for Malaria Venture, heavily funded by the Gates Foundation, has worked with pharmaceutical company Novartis (NVS) to develop good-tasting malaria pills and distribute them to millions of children -- the principal victims of the disease -- in 24 countries.
Another fact about the 2006 Buffett article is that it was written by yours truly, Carol Loomis, a senior editor-at-large of Fortune. Besides that, I am a longtime friend of Buffett's and editor of his annual letter to Berkshire's shareholders. Through him, my husband, John Loomis, and I have also come to know Melinda and Bill Gates socially. The Loomis team has even occasionally played bridge against Warren and Bill.
All that said, the question of what philanthropy might gain from the Gates/Buffett drive rests, at its outset, on a mystery: what the wealthiest Americans are giving now. Most of them aren't telling, and outsiders can't pierce the veil. For that matter, the Forbes 400 list, while a valiant try, is a best-guess estimate both as to the cast of characters and as to their net worth. (Buffett says he knows of two Berkshire shareholders who should be on the list but have been missed.) As Bill Gates sums it up, "The list is imprecise."
Those qualifiers noted, the magazine stated the 2009 net worth of the Forbes 400 to be around $1.2 trillion. So if those 400 were to give 50% of that net worth away during their lifetimes or at death, that would be $600 billion. You can think of that colossal amount as what the Buffett and Gates team is stalking -- at a minimum.
Leaving aside the Forbes 400 and looking simply at Internal Revenue Service data for both annual giving and estate taxes, we can piece together a picture of how far the very rich might be from a figure like that $600 billion. Start with an admirable fact about Americans as a whole: The U.S. outdoes all other countries in philanthropic generosity, annually giving in the neighborhood of $300 billion.
Some of that gets reported as charitable deductions on the tax filings made by individuals. But taxpayers at low income levels don't tend to itemize, taking the standard deduction instead. At higher income levels, charitable gift data begin to mean something. To take one example for 2007 (the latest data available), the 18,394 individual taxpayers having adjusted gross income of $10 million or more reported charitable gifts equal to about $32.8 billion, or 5.84% of their $562 billion in income.
And billionaires? Here, the best picture -- though it's flawed -- emerges from statistics that the IRS has for almost two decades been releasing on each year's 400 largest individual taxpayers, a changing universe obviously. The decision of the government to track this particular number of citizens may or may not have been spurred by the annual publication of the Forbes list. In any case, the two 400 batches, though surely overlapping, cannot be identical -- for one reason because the IRS data deal with income, not net worth.
The IRS facts for 2007 show that the 400 biggest taxpayers had a total adjusted income of $138 billion, and just over $11 billion was taken as a charitable deduction, a proportion of about 8%. The amount deducted, we need quickly to add, must be adjusted upward because it would have been limited for certain gifts, among them very large ones such as Buffett's $1.8 billion donation that year to the Gates Foundation. Even so, it is hard to imagine the $11 billion rising, by any means, to more than $15 billion. If we accept $15 billion as a reasonable estimate, that would mean that the 400 biggest taxpayers gave 11% of their income to charity -- just a bit more than tithing.
Is it possible that annual giving misses the bigger picture? One could imagine that the very rich build their net worth during their lifetimes and then put large charitable bequests into their wills. Estate tax data, unfortunately, make hash of that scenario, as 2008 statistics show. The number of taxpayers making estate tax filings that year was 38,000, and these filers had gross estates totaling $229 billion. Four-fifths of those taxpayers made no charitable bequests at death. The 7,214 who did make bequests gave $28 billion. And that's only 12% of the $229 billion gross estate value posted by the entire 38,000.
All told, the data suggest that there is a huge gap between what the very rich are giving now and what the Gateses and Buffett would like to suggest is appropriate -- that 50%, or better, of net worth. The question is how many people of wealth will buy their argument.