Although the financial markets have been on the upswing recently from their post-boom low, many of the nation's private pension plans have been sinking deeper into the hole, according to new figures from the government's pension insurance agency.

The 1,108 weakest pension plans -- those whose assets are at least $50 million below the value of the benefits they promise -- were short by an aggregate $353.7 billion at the end of last year, figures from the government's Pension Benefit Guaranty Corp. show. That was 27 percent more than the shortfall a year earlier, contrary to the hopes of many that funding would improve as the economy strengthens.

House and Senate committees are looking for a way to shore up the pension agency and the plans it insures without driving more employers out of the traditional pension system.

The quest took on new urgency in recent month because of the declining financial health of the PBGC, which is itself underfunded by some $23.3 billion, and questions about the future of the Social Security system, which is the sole source of retirement income for about 20 percent of the nation's elderly.

The Bush administration this year offered a plan that would sharply increase premiums charged to employers that operate traditional pensions. The plan would also tighten pension funding requirements, especially for financially weak employers.

Employer groups say Bush's plan would raise the cost of pensions more than necessary, encouraging employers to terminate even healthy plans.

The Senate Finance Committee and a subcommittee of the Senate Health, Education, Labor and Pensions Committee are holding hearings today on various aspects of the pension system and Thursday on protecting pensions from fraud. On Thursday, the House Budget Committee is holding a hearing on the PBGC's liabilities. And Rep. John A. Boehner (R-Ohio), chairman of the House Committee on Education and the Workforce, is working on a pension bill that he expects to introduce soon, perhaps this week.

"The level of underfunding in worker pension plans today is the consequence of outdated laws that present a danger to workers, retirees and taxpayers," Boehner said yesterday. "Without fundamental reform, more companies will default on their worker pension plans, increasing the likelihood of a multibillion-dollar taxpayer bailout, and more companies will stop providing defined-benefit pension plans to their workers altogether."

The number of traditional pensions has fallen from more than 100,000 to about 31,000 over the past two decades. And though the remaining plans still cover 44 million workers and retirees, a growing number of workers and families now have only retirement savings plans, such as 401(k)s, and other savings for retirement.

In a 401(k) plan, there is no promised benefit; whatever is in the account at retirement is what the retiree gets.

In traditional pensions, benefits are promised and employers generally bear the burden of funding them. Employers, with the PBGC as a backstop, are supposed to make up the shortfall if the pension fund's investments do not do well.

But the end of the stock market boom, and changes in the fundamental economics of entire industries, such as airlines and steel, have left many plans underfunded and have thrust others into the arms of the PBGC. Also, loose funding rules allowed companies -- more than three in five of the largest ones each year from 1995 to 2002 -- to avoid putting new money into their plans, a study by the Government Accountability Office found.

"A company's pension promise is only as strong as the rules that require companies to fund these plans, Senate Finance Committee Chairman Charles E. Grassley (R-Iowa) said yesterday. "Unfortunately, we're finding that the rules themselves are full of very serious holes and that the hole at the PBGC could become a crater."

Grassley said he wants to look at the PBGC's deteriorating finances, "how we got here," and what can be done.

The rise in underfunding shows how difficult it is for companies to return pension plans to funding levels of the boom years of the 1990s. In 1999, for example, only 166 plans were underfunded by $50 million or more, and the total shortfall was $18.36 billion. By 2002, 1,058 plans were $305.88 billion under, but in 2003 there was a modest upturn as 1,050 plans listed a total shortfall of $278.57 billion. That did not last, however.

"Despite a strong economy, pension plans are reporting even larger shortfalls than last year. Clearly we need new rules that will require companies to fund the pension promises they make," PBGC Executive Director Bradley D. Belt said yesterday.

Not all underfunded plans are reflected in the latest figures. Some are too small to have $50 million in underfunding, and some escape because of the way the filing requirement is structured. In September, the PBGC estimated the total underfunding in all insured pension plans at more than $450 billion.

The government guarantee is limited to a maximum of $45,614 in annual benefits for a retiree age 65 or older. Workers who were promised higher benefits, such as airline pilots, do not get their full pensions if their plans are taken over by the PBGC. That limits the agency's liability but hurts retirees.

Rep. John A. Boehner (R-Ohio ) plans to introduce a pension bill in the House soon.