The private-equity firms Warburg Pincus and Thomas H. Lee Partners are selling roughly a third of their combined stake in Umpqua Holdings of Portland, Ore.

On Wednesday the $22 billion-asset Umpqua announced the start of a secondary offering of 15 million shares owned by the private-equity firms. It did not specify how much each firm is selling; Warburg and Lee Partners each own about 21.6 million shares.

Their Umpqua shares are tied to Sterling Financial of Spokane, Wash., which Umpqua acquired for roughly $2 billion this year. The two private-equity firms led a $730 million recapitalization of Sterling in 2010.

The deal called for Sterling shareholders to receive 1.671 Umpqua shares and $2.18 cash for each share owned.

As a condition of the deal closing, Umpqua registered the 43 million shares owned by the two firms, which makes it easier for them to sell their stakes.

Jeff Rulis, an analyst with D.A. Davidson, said the private-equity ownership — and questions over its potential exit — has likely hurt Umpqua’s stock price. The stock was trading at $18.48 when the deal closed in April, and then fell 13% in the following month. The company beat second-quarter expectations by four cents and the stock has come back a bit.

On Wednesday, the stock closed at $16.69, up less than 1%; it had fallen 1.5% in after-hours trading early in the evening.

Ron Farnsworth, the chief financial officer of Umpqua, acknowledged that some investors had been concerned about the threat of dilution, but he said he expects the offering to be a "positive development" for the stock price.

Farnsworth said questions about why Warburg and Lee Partners are selling the shares now should be directed to the firms, adding that the nature of private equity is often to gradually sell its holdings.

The decision to float only a piece of the stock was made by the two firms and the underwriters of the offering, Farnsworth said. JPMorgan is underwriting the offering.

"They do it in a way to make sure there is little volatility brought on our stock," Farnsworth said.

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