Deutsche Bank is Germany's largest bank
Deutsche Bank has issued a profits warning, saying it made an estimated loss of 4.8bn euros ($6.4bn; £4.4bn) in the fourth quarter of last year.
It said the loss reflected "exceptional market conditions", which hit its sales and trading businesses.
The bank, Germany's largest, now says it expects its full-year loss for 2008 of about 3.9bn euros.
Chairman Dr Josef Ackermann said: "We are very disappointed at this fourth quarter result."
Dr Ackermann added: "The exceptionally difficult market environment of the quarter exposed some weaknesses in our platform, and we have determined a number of measures to address these weaknesses.
We have substantially reduced our exposures in leveraged finance, commercial real estate and other key credit market exposures
Dr Josef Ackermann, Deutsche Bank
"Implementation of these measures is already under way."
Deutsche Bank's fourth-quarter and full-year earnings are due to be released on 5 February.
Despite the tough conditions, Deutsche Bank is buying a stake in Postbank.
On Wednesday, it was announced that, according to a new deal between the banks, Deutsche Bank will now buy 22.9% of Postbank.
Deutsche Bank so far has not made use of the German government's financial sector rescue fund, which has a total value of up to 500bn euros.
The bank said that as well as some of the "corrective adjustments" decided upon by the management board, more would follow in 2009.
"Our capital strength, which we have successfully maintained, allowed us to withstand these extremely difficult market conditions and to take necessary steps to de-risk our platform," Dr Ackermann added.
"We have substantially reduced our exposures in leveraged finance, commercial real estate and other key credit market exposures, and expect no further material negative impact from these areas."
He also said the bank had reduced exposure to, or exited, trading strategies that were most affected by current market turbulence.
"We have significantly reduced trading assets, and thus reduced balance sheet leverage," he said.