NEW YORK — A closely watched index shows home prices tumbled by the sharpest annual rate ever in August.

The Standard & Poor's/Case-Shiller 20-city housing index released Tuesday dropped a record 16.6 percent in August from the year-ago month, the largest drop since its inception in 2000. The 10-city index plunged 17.7 percent, its biggest decline in its 21-year history.

Both indices have recorded year-over-year declines for 20 consecutive months.

The index also fell in Dallas, but only by 2.7 percent over the 12-month period ending in August. The biggest year-to-year price dip in Dallas was recorded in February, with a 4.1 percent decline.

The Case-Shiller index for North Texas had increased every month from the date it was first published — in January 2000 — until October 2007, when it began to decline.

Prices in the 20-city index have plummeted more than 20 percent since peaking in July 2006. The 10-city index has fallen nearly 22 percent since its peak in June 2006.

No city in the Case-Shiller 20-city index saw annual price gains in August — for the fifth straight month.