Madison Dearborn Partners LLC, a Chicago-based private equity firm, is acquiring a majority 51% interest in credit reporting company, TransUnion LLC.

TransUnion "is a market leader in an industry that will continue to play a critical role in the global economy: helping businesses to better manage risk and improve decision-making, and consumers to understand and manage their credit," Tim Hurd, Madison Dearborn managing director, said in a statement.

The Pritzker family will keep about 49% ownership in TransUnion. Terms weren't disclosed.

TransUnion is the third-largest credit bureau, according to analysts. The largest, Experian, which has credit information on an estimated 300 million consumers, reported 2009 revenue of $3.9 billion. No. 2 Equifax reported revenue of $1.8 billion last year and information on more than 400 million credit holders worldwide. It has a market capitalization of $4.2 billion.

Privately held TransUnion doesn't report results, but an industry analyst with Stephens Inc., Carter Malloy, estimated its annual revenue at more than $1 billion. Applying a valuation similar to that of Experian and Equifax would imply TransUnion, which has information on 500 million people worldwide, is worth between $2 billion and $3 billion, a source told the Chicago Tribune.

Private-equity firms such as Madison Dearborn have seen the debt markets open up wide, a change from 2008 and 2009.

TransUnion Chair Penny Pritzker, in a statement, said: "We look forward to partnering with the Madison Dearborn team to capitalize on the numerous growth opportunities before us."

The 11 heirs of Jay Pritzker, who died in 1999 after building a portfolio estimated at $15 billion, reached an agreement in late 2001 to divest the family's assets by 2011.

In the most recent major divestiture, the Pritzkers took Hyatt Hotels Corp. public last November, raising $950 million.

In March 2008, the family sold a majority stake in the Marmon Group to Berkshire Hathaway Inc. for $4.5 billion. In 2006, the family sold Conwood, a smokeless tobacco company, for $3.5 billion.

Madison Dearborn's other noteworthy financial services investments include Nuveen Investments, CapitalSource and PayPal.

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