The prices of existing homes declined at an unprecedented annual rate during the first half of this year, according to the Standard & Poor's/Case-Shiller home price indexes.

In August, existing home prices were down 17.7% from a year earlier in the 10-city index, and down 16.6% in the larger 20 city index, a slight increase from the July rate of decline.

"The downturn in residential real estate prices continued, with very few bright spots in the data," said David Blitzer, chairman of the index committee at S&P.

He noted that for the fifth straight month, every region of the country posted declines. Both the 10 city and 20 city composite indices have been declining on a year-over-year basis for 20 straight months.

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