Three years of plunging stock prices and rising benefit liabilities have thrown many of the largest pension plans in corporate America deeply into deficit -- a situation that will siphon away billions of dollars in cash and reported earnings in the next few years unless the market rebounds sharply.

That's the picture painted by a survey released yesterday, covering 100 of the largest corporate pension plans, by Milliman USA, a benefits consulting firm. It is based on figures included in companies' annual reports.

The plans, run by such giants of industry as General Motors Corp., International Business Machines Corp., Boeing Co. and AT&T Corp., have been hit by "a perfect storm" of declining asset values and declining interest rates, said John W. Ehrhardt, an actuary with Milliman. Lower interest rates caused liabilities to rise because of the way their value is calculated, and falling stock prices reduced the assets available to pay them.

The report shows that collectively these companies' pension plans went from a funding surplus of $183 billion at the end of 2000 to a deficit of $157 billion at the end of 2002. Together their assets dropped from 124.5 percent of their liabilities to 82.4 percent.

The largest deficit it found, $25.4 billion, was at General Motors, which last year contributed $5.1 billion to shore up its plan. Others with large deficits included Ford Motor Co. with $15.6 billion, Exxon Mobil Corp. with $11.3 billion, IBM with $6.4 billion and Delta Air Lines with $4.9 billion.

Some of the most underfunded on a percentage basis were Procter & Gamble Co., where assets equaled 44.8 percent of liabilities; ConocoPhillips Co., 49.3 percent; and Exxon Mobil, 50 percent.

Milliman did not call the deficits it found a threat to the system, but its experts said they are enough to put a squeeze on profits and cash flow at many companies this year.

For much of the 1990s, pension funds at many of these companies produced hefty investment returns that companies were allowed under accounting rules to include in their earnings. They cannot actually take the cash out, but can record a negative pension "expense" that adds to the bottom line.

The report comes at a time of increasing focus on pensions and pension accounting. Employers are seeking ways to minimize plan costs, including legislation to change requirements for calculating certain liabilities to make it easier for them to comply with funding requirements.

Also, there is growing debate about the assumptions used when pension-fund investment returns are included in companies' profit statements. The Securities and Exchange Commission has raised questions about firms' optimistic return assumptions that turned pension plans into profit centers for some companies in the 1990s.

And underfunded pension plans in certain industries, notably steel and airlines, have raised questions about the ability of the government's pension insurance agency, the Pension Benefit Guaranty Corp., to meet its obligations in future years.

The report referred to defined-benefit pensions, in which benefits are based on a formula, often involving pay and years of service, and the employer bears the investment risk.

"We were in the unusual position during the '90s -- and many companies are still there -- that you didn't have an expense, you had a credit coming out of the pension expense calculation," Ehrhardt said.

"That has just about been eliminated," he said, adding that he is "very confident" that for these companies in 2003, the pension income, which was $3.3 billion in 2002, "is going to be more than reversed, down to easily a $3 [billion] to $5 or more billion charge."

It is already happening for some companies, he noted. For example, Bank of America went from $38 million in pension income in 2000 to a $140 million cost in 2002.

Pension plans are allowed to make assumptions about their future earnings and plug those numbers into its earnings. Those assumptions have been unrealistically high in recent years, with many companies continuing to use 10 percent as the market produced negative returns.

Now companies are cutting those assumptions -- more than half have already done so -- so that the average for this group was 8.92 percent for 2002, down from 9.36. When those reduced percentages are applied to shrunken asset bases, the effect is a sharp reduction in the pension-return figures that are taken into the company's profit-loss calculations.

And some experts think the expectations remain too high. Investor Warren Buffett has said that he thinks 6.5 percent is closer to reality.