At a glance, corporate pensions might appear to be climbing up off the mat.

After all, just a few years ago companies in the Standard & Poor's 500 Index ($INX) had pension plans underfunded by $203 billion. Now that shortfall has shrunk to $77 billion.

But don't be fooled. Pension plans are getting healthier for one big reason: rising stock prices. But just because stocks have been rising for the past four years doesn't mean the trend will last.

"Everyone is talking about how pension plans are healthy now," says David Zion, an accounting expert who follows pension issues for Credit Suisse. "But wait a minute. We just saw this movie a few years ago."

Zion is referring to the fact that the stock-market rally of the late 1990s helped S&P 500 companies boast a $239 billion overfunding of pensions -- until the subsequent market meltdown made that surplus go poof. When that happened, companies in the S&P 500 had to increase their annual pension contributions from $10 billion in 1999 to $71 billion in 2003.

Another stock-market slide almost certainly would create similar problems. The question is, which companies look most likely to lose if that happens?

Big plans, big risks
One way to find companies with looming problems is to compare the size of their pension obligations with their market capitalizations. The bigger the imbalance, the bigger the risks. At Ford (F, news, msgs), for example, promised pension payouts are nearly five times the value of the company. Last year, pension promises were $75.8 billion worldwide, compared with a recent market cap of $16.1 billion.

Unisys (UIS, news, msgs) and Goodyear Tire & Rubber (GT, news, msgs) stand out as other S&P 500 companies with the biggest imbalances, with pension promises nearly two times their recent market caps (see box).