Now that hedge funds all have access to the same technology and strategies, they have fewer obvious advantages. This is one reason Griffin is working to transform Citadel into a more traditional financial-services company, offering technology and administrative support to other funds. "It seems as though a lot of owners of hedge fund institutions, especially ones that have been around awhile, are asking themselves whether they want to have an organization that lives beyond them," says Erin Callan, the global head of hedge fund coverage at Lehman Brothers. "This is a question that hasn’t been addressed before."

So Griffin watched this year’s $600 million public offering by Fortress Investment Group, a hedge fund and private equity firm, with great interest. On its first day of trading, in early February, the stock shot up 68 percent, to close at $31 per share.

Wall Street is convinced that Griffin is plotting an I.P.O. for Citadel, particularly after the Blackstone Group announced plans last month to sell shares to the public. The news has stirred up interest among a number of hedge funds and private equity firms, whose managers now worry that the industry’s boom may be ending. Citadel’s sale last December of $500 million in bonds did little to dispel the speculation: Creating a 363-page pitch book filled with previously undisclosed information about the company and marketing the bonds to an unusually wide range of investors screamed "I.P.O. dry run" to many. Griffin plays coy about his intentions: "It’s a strategic option we consider," he says, regarding the possibility of taking Citadel public. "But it’s a strategic option. It’s not a foregone conclusion."

Nor is the industry’s continued success. A research report by Dresdner Kleinwort highlights some of the challenges facing Griffin and his competitors. In an analysis of the amount of debt being built up by many hedge funds, the authors worry that the funds could pose "systemic risk" to the financial markets. In one sense, the report does Griffin a favor, comparing Citadel with an investment bank. But because Citadel and other hedge funds are lightly regulated, the report says that Citadel’s leverage—its total assets to capital—could be a concern, even though it’s about half the level that Goldman Sachs had recently.

"At face value, and without being able to look into the black box, the balance sheet of today’s Citadel hedge fund looks quite similar to L.T.C.M.," the report states. It goes on to suggest that hedge funds such as Citadel might need greater regulation—something Griffin opposes.

Anne Dias Griffin is the champagne to Ken Griffin’s Budweiser. Dias was born near Strasbourg, France, where she attended an international school and learned English, Spanish, and German. Stylish and delicate, with highlighted blond hair and wide brown eyes, she exudes a chilly air of unattainability.

After high school, Dias studied at the foreign-service school at Georgetown University, and during one summer she interned at the Financial Times in London, thinking that she might become a journalist. (She wrote a front-page -story on Brazilian debt-for-nature swaps for the paper.) After graduating, Dias entered the analyst program at Goldman Sachs. She went on to Harvard Business School and worked at Soros Fund Management and Viking Global before starting her own fund with seed money from hedge fund pioneer Julian Robertson. (Her fund is reported to have $55 million in assets; Dias says the number is higher but would not say by how much.)

At first glance, Griffin and Dias appear to be utterly different. In contrast to her sophistication, he enjoys such red-blooded pursuits as driving race cars, playing the occasional game of Texas Hold ’Em, and competing in weekend-warrior soccer leagues. But on closer examination, their pairing makes perfect strategic sense, in the same way a merger might be characterized by enthusiastic investment bankers as having "synergies" or "economies of scale."

"Ken is really a pure American boy," says Tom Pritzker, chairman of Hyatt International and head of the Art Institute of Chicago’s board of trustees. "But Anne brings a European flair and style to the couple."

For the past several years, she has been an important factor in raising Griffin’s public profile. They married in 2003 in the gardens of Versailles, outside of Paris. (Griffin’s first marriage ended in divorce in 1994.) He and Dias live in a $6.9 million penthouse on Chicago’s Gold Coast.

They tend to speak in unison and are fond of using the word passion. Such as when Griffin says, "We are, with time, coming to terms with what we’re passionate about in the way of our involvement in the community." Or Dias, on the subject of collecting: "I’ve always been passionate about art"—to which Griffin responds, "It was an unexpected surprise in the context of the relationship that we shared this passion."