A fourth nationwide measure of U.S. home values issued by Seattle-based Zillow.com shows a 1.7% sequential drop from the first to the second quarter of this year and a 9.9.% drop from last year's second quarter, the biggest decline in 12 years.

Zillow's Home Value Index (HVI) now stands at $206,919, the company said Tuesday. The median U.S. home value is now back at fourth-quarter, 2004 levels, the company said, which leaves 29.1% of homeowners who purchased since 2003 with negative equity.

The company claims its monitoring of 165 MSAs gives it the largest sample of any housing report. The declines it reports fall closer to those reported by S&P/Case-Shiller and the National Association of Realtors than they do to those put out by OFHEO, which tracks only homes financed by conforming mortgages.

Zillow said Tuesday that 140 of its surveyed MSAs lost value since the second quarter of 2007 and that homes in several markets, includng Stockton, Las Vegas, Los Angeles and Miami have lost more than 20% of their value in just the past 12 months.

Zillow estimates the highest rates of negative equity are among those who purchased in 2006, when most markets peaked, as nearly half (45%) of those buyers across the U.S. now face negative equity after placing a median down payment of 10%. In all, 32.7% of homes sold in the second quarter were sold for a loss and 18.6% were foreclosure sales compared to 12.2% and 7% respectively in last year's second quarter. Nearly one in four (23.7%) homes sold during the past year sold for a loss while nearly 15% of sales were foreclosures.

"The second quarter is the sixth consecutive quarter of home value declines and we see little promise of turnaround in the short-term as the rates of decline have yet to slow and, in fact, actually accelerated in many markets," said Dr. Stan Humphries, Zillow's vp of data and analytics. "The high rates of negative equity are having a direct effect on home sales figures as we've seen considerable growth in foreclosure transactions and homes selling for a loss."

Based on the Zillow Q2 Real Estate Market Reports, 77% of homes actually declined in value over the past year, a slight increase from the 75% that declined year-over-year in the first quarter.