NEW YORK (Fortune) -- Countrywide on Tuesday reported a loss of $422 million in the fourth quarter and revealed that an astounding one-third of its investment portfolio's sub-prime mortgage loans are delinquent.

The loss threw cold water on Countrywide chief operating officer Steve Sambol's confident assurances to investors in October that, "We view the third quarter of 2007 as an earnings trough, and anticipate that the company will be profitable in the fourth quarter and in 2008." Seen in this light, Countrywide's fourth-quarter loss, compared to a $621 million profit a year ago, is what the numerous class action attorneys circling Countrywide (CFC, Fortune 500) will surely call "an unfavorable fact." Countywide finished 2007 with a loss of $704 million.

The numbers didn't appear to faze Bank of America CEO Ken Lewis's determination to acquire Countrywide, however. In a conference Tuesday, Bloomberg quoted him as telling investors. "Everything is a 'go' to complete this transaction." Just over two weeks ago, BoA (BAC, Fortune 500) agreed to buy Calabasas, Calif.-based Countrywide in a $4 billion deal. If and when the deal goes through, the combined company will control just over 25 percent of the U.S. real estate loan origination market.

The market took the highly scripted BoA support as crucial and sent Countrywide stock up 20 cents to $6.15.

At the fulcrum of the mortgage credit crisis, Countrywide's earnings are seen as a bellwether for the once vibrant - and now largely collapsed - United States mortgage industry. The primary culprit remains a combination of old-fashioned credit deterioration plus an alarming new development: Borrowers simply are walking away from their homes as their equity value falls ever further below their loan amount.

With respect to credit problems, Countrywide is unmistakably going from bad to worse.

The home lending giant reserved $924 million for credit-related losses in the fourth quarter - over a dozen times more than what it set aside in the fourth quarter of last year. To be fair, the $924 million figure is a bit of an improvement from the third quarter's $937 million reserve.

Countrywide's eye-popping 33% delinquency rate on its sub-prime mortgage book also represents a decline from the third quarter, where "only" 29.6% of sub-prime paper was delinquent.

The figures obscure a central fact, however: Countrywide's portfolio of sub-prime loans consist of those that were not previously written down, or could not be sold or securitized. In other words, this portfolio is likely to get much, much worse.

Perhaps as worrisome was the credit deterioration on the conventional loan portfolios, where delinquency rates spiked to 5.76% percent from 4.41%. Along similar lines, Countrywide also said it shifted $7 billion of "prime non-agency" loans to the portfolio - out of the held-for-sale inventory - because it appears that there were no buyers likely to be found for this paper.

Adding to those woes is the emergence of a disturbing trend for mortgage lenders as some borrowers choose to simply walk away from their homes.

As home values drop to levels far below the mortgage amount, it simply becomes more economically advantageous for certain borrowers to hand over their keys. In a foreclosure, a mortgage lender often winds up booking losses that approach or even outstrip its loan amount when it sells the property into a distressed market.