The number of Fortune 1000 firms that have a frozen pension plan increased from 169 in 2008 to 190 in 2009, according to an analysis by Watson Wyatt, a consulting firm. In 2004, 45 companies had a frozen pension plan.

Overall, the rate of pension-plan sponsorship in the Fortune 1000 decreased slightly from 63 percent in 2004 to 61 percent in 2009. However, almost half of the pension-plan sponsors in the financial services industry and one third of the pension-plan sponsors in the automobile industry have frozen plans.

The analysis shows that employers continue to look for ways to control their retirement benefit expenses.

"As companies in industries that have suffered through the crisis struggle to keep their heads above water, freezing or closing pension plans may seem like an effective way to cut costs," said Alan Glickstein, senior retirement consultant at Watson Wyatt. "However, this strategy could come with substantial hidden costs -- for employers, who could face increased difficulties in managing the retirement of their workforces, and for employees, who could face reduced retirement resources as a result of a frozen pension plan or a reduced 401(k) match."

A separate study by Watson Wyatt found that freezing a pension plan does not lead to a boost in a company's stock price.