CHICAGO—Eighteen months after retiring as executive chairman of Aon Corp., Patrick G. Ryan is embarking on a new venture that he plans to build into a significant player in the wholesale insurance marketplace.

Ryan Specialty Group will serve as a global holding company of managing general underwriters, managing general agencies and brokerages, and will provide specialty insurance services to retail agents and brokers around the world.

The announcement of Chicago-based RSG follows the December formation of ThinkRisk Underwriting Agency L.L.C., a Kansas City, Mo.-based managing general underwriting agency specializing in errors and omissions coverage for the media, advertising, technology, privacy and network security industries.

ThinkRisk, RSG's first subsidiary, was founded by industry veterans Leib Dodell, Debra McClenahan and Chad Milton, all of whom have ties to Aon. Mr. Ryan is chairman of the MGU, while Mr. Dodell is CEO.

RSG last week also announced the launch of its second subsidiary, startup Ryan Specialty (Europe) Ltd., a U.K.-based MGU specializing in financial lines products, including directors and officers, financial institution and professional liability products.

Malcolm Nightingale and Adam Barker, formerly of HCC Global Financial Products L.L.C., have joined RSG to lead the new MGU.

Also joining RSG last week was Timothy W. Turner, former president of the nation's largest wholesaler, CRC Insurance Services Inc., who is a managing director of the holding company.

In an interview, Mr. Ryan, 72, said even though he retired from active involvement with Aon in 2008, he never intended to fully retire from the insurance industry.

Although in the interim he took on the role of CEO of Chicago 2016, the city's unsuccessful bid to host the 2016 Summer Olympic Games, Mr. Ryan said he would not have stayed on in that capacity had Chicago won.

“I thought I'd get back into the insurance business,” Mr. Ryan said, but not in a capacity that would compete with Aon, of which he remains the largest individual shareholder.

Toward that end, RSG will not be in the retail brokerage, reinsurance brokerage or human resource consulting spaces, nor will it compete in the same lines of business as Aon's existing MGU operations, stressed Mr. Ryan, who is chairman and CEO of RSG.

Rather, he said he intends to build a specialty wholesale company that addresses new and emerging needs.

It's a segment of the business that is familiar to Mr. Ryan, who in 1964 established Pat Ryan Associates, a small MGU that distributed specialty credit life insurance to automobile dealers. Mr. Ryan ultimately built the agency into Aon, the world second-largest brokerage firm, according to BI's rankings.

“It's not by coincidence,” Mr. Ryan said, that he's back in the MGU/MGA business. “I've always liked that sector. I believe that it does allow you to really demonstrate your creativity” and differentiation, he said.

Indeed, “Pat Ryan is a leader in the industry and it will benefit from his vision,” an Aon spokesperson said of the launch of RSG.

Other industry observers say they wouldn't bet against RSG's success.

“There's plenty of room for creative solutions and competition” in the industry, said Ken Crerar, president of the Council of Insurance Agents and Brokers in Washington. “Pat Ryan is surely one of the icons of the industry in terms of building a major brokerage operation and it will be interesting to watch and see what he does in this realm.”

“I think he's going to be a major player in the wholesale, MGA, MGU space very quickly,” said Kevin P. Donoghue, managing director of Mystic Capital Advisors Group L.L.C. in New York, specifically noting the “awesome team” of “extremely competent” people who have joined RSG so far. “He's not going to put his name behind something without succeeding.”

Mr. Donoghue added that he would not be surprised if RSG ultimately “picked up” one of the large, private equity-backed wholesalers whose owners may be looking for an exit strategy.

Wholesalers Crump Group Inc., AmWINS Group Inc. and Swett & Crawford Group are entering the fifth year of their respective private equity-ownership structures.

When asked to comment on persistent rumors that he might be interested in purchasing Swett & Crawford, the wholesale operation that Mr. Ryan helped divest from Aon in September 2005, he laughed and said, “There are a lot of rumors out there. Swett and Crawford is a great company.”

Mr. Ryan did say that he plans to build RSG into a company with meaningful scale through a series of acquisitions, startups and “bolt-on” acquisitions to those startups.

He noted that he's had “quite a bit of interest from outside investors, so capital is not an issue.” It all depends “on how quickly we can grow prudently,” he said.