The Mercer analysis estimates that the aggregate funded level of plans sponsored by companies in the S&P 1500 at the end of November was 80 percent, down from 97 percent at the end of September and 104 percent at the end of 2007.
"October and November were particularly bad months for pension plans. Falling equity values and falling corporate bond yields have resulted in the sharpest decline in funded status in more than a decade," Adrian Hartshorn, a Mercer principal in New York, said in a statement.
In all, pension plan liabilities of companies in the S&P 1500 exceeded assets by $280 billion. That compares with a surplus of $60 billion at the end of last year.
The dramatic drop in plans' funded status comes as employers are banding together to lobby Congress to provide temporary relief from tough funding requirements set by a 2006 federal law. Among other things, that law requires employers to fund pension deficits over a seven-year period.