The federal agency that guarantees pensions has lost $2.1 billion on its investments so far this year, foreshadowing expected losses among corporate pension funds, state retirement systems and others that provide a financial backstop for an aging population.

The loss at the agency, the Pension Benefit Guaranty Corporation, was magnified by its decision in February to invest more aggressively to narrow its deficit. With unemployment rising and more companies coming under financial pressure, the agency will undoubtedly face more claims from companies that cannot make good on their pension promises.

The stock portion of the agency’s roughly $68 billion investment portfolio fell in value by $2.2 billion through August. Small gains in the fixed-income instruments offset part of the loss. These results do not include the devastating market swings of September.

Representative George Miller, the California Democrat who spotted the decline, said it stood as a warning about what could happen to any body of government if officials decided to invest the funds backing up a safety-net program in volatile securities.

“There has only been one guarantee in this financial crisis, and that is that Social Security did not lose money in the market,” said Mr. Miller, the chairman of the House Committee on Education and Labor. “The current market turmoil is proof that we should not subject our basic retirement security to a riverboat gamble.”

The information about the losses came to light as the committee prepared for a hearing on Friday on jobs and the economy. Mr. Miller said he had added to the list of those who would appear the executive director of the agency, Charles E. F. Millard, so that the committee could ask him about the its decision to put more money into equities. He recalled that Mr. Millard had previously told Congress that switching to more stocks would not add risk.

Mr. Millard said in an e-mail message that the agency’s losses were “well below the losses suffered by most other investors.”

“Our investment managers will continue their careful approach as they diversify our portfolio and seek long-term market opportunities,” he added.

A spokesman for the pension guarantor, Jeffrey Speicher, said that the losses should not alarm the workers and retirees counting on the agency for their pensions. “Participants are no worse off,” he said. “We still have plenty of money to meet our commitments.”

The agency maintains its investment pool to pay the pensions of workers and retirees whose companies have gone bankrupt. Some of the money comes from insurance premiums paid by companies that sponsor traditional pension plans; that portion of the pool is invested in Treasury bonds. The other assets are acquired when the agency takes over pension funds. Traditionally, those have been invested heavily in equities and remained in equities at the agency.

The agency itself has operated at a deficit almost consistently from the time it was created in 1974. Its finances took a particularly severe beating during the bear market at the beginning of this decade, when a series of large steel companies and airlines went bankrupt and sent their pension funds to it as they struggled to reorganize.

By 2004, the agency decided it was a mistake to keep its portfolio so heavily in stocks, because they tended to lose value just when stock-laden corporate pension funds were themselves likeliest to fail — which was precisely when the insurer most needed the money.

So the agency announced that it was scaling back on stocks and bolstering its holdings of fixed-income instruments. Fixed-income securities, like bonds, can also swing in value, but they can provide a predictable stream of income that matches the agency’s own payout schedule.

But as memories of the bear market faded and the leadership of the agency changed, the bond-based strategy lost favor. Stocks are thought to yield more over the long term, and Mr. Millard said he thought they would be a better tool for closing the agency’s deficit, which was $14 billion last year.

Despite the investment loss this year, Mr. Millard said, “we expect our 2008 deficit will be about $2 billion lower than last year’s.”