NEW YORK (CNNMoney.com) — May home prices dropped a record 15.8% from a year ago, according to the S&P/Case-Shiller Home Price Index of 20 cities. It was the 22nd consecutive month of decline recorded by the index. Prices fell 0.9% from April to May.

Each of the 20 metro areas covered by the index posted annual declines; nine posted record lows and 10 cities recorded double-digit drops.

The Case-Shiller 10-city Index posted a year over year decline of 16.9%, and a 1% month over month dip. Both the 10-City Composite Index and the 20-City Composite Index are reporting record annual declines.

“Since August 2006, there has not been one month where we have seen overall price increases, as measured by the two Composites,” said David Blitzer, Chairman of the Index Committee at Standard & Poor’s.

Case-Shiller has been tracking the 20-city index for 19 years, while the 10-city index is 21 years old. The current streak of price declines has been unprecedented in both its length and depth. The last extended decline began in in April 1990, when the 10-city index sank for 10 consecutive months. But that total loss was just 6.5%.

Since the 10-city index peaked in July 2006, it has plunged 19.8%. The 20-city is down 18.4% from the peak.

The 20-city index’s Sun Belt cities, which recorded the biggest price gains during the boom, have led the charge down. Las Vegas prices have plummeted 28.4% during the past 12 months; Miami prices fell 28.3%; and Phoenix homes lost 26.5% of their value.

Midwest metro areas, which have endured tough economic times for years, are also feeling the pain. Detroit prices are off 17.4% for the 12 months, and Cleveland is down 8%.

Northeast cities like Boston, down 6.2% for the 12 months, and New York, off 7.9%, have been less volatile than the Sun Belt.

The smallest year-over-year declines were recorded by Charlotte, N.C. (down 0.2%), Dallas (down 3.1%), and Denver (down 4.8%).

The soaring numbers of foreclosures are helping to push down prices. Banks tend to slash prices when selling repossessed homes, since they lose money every month a house sits vacant. They must pay property taxes, maintenance expenses and utility costs while getting nothing back in return.

Those sales, in turn, tend to bring down prices in the rest of a given neighborhood, creating a vicious cycle.

Foreclosures accounted for a large - and growing - share of all existing homes sold in some markets. In California, for example, 40% of the existing homes sold during the three months ended June 30 were foreclosures, according to DataQuick, a real estate information provider. That’s up from just 5.4% during the same period in 2007.

Optimistic observers might point out that price declines appear to be slowing. The 10-city index’s 1% month to month dip in May was less than April’s, when it registered a 1.5% decline, while the 20-city index fell just 0.9% in May after dropping 1.3% in April.

Can this be a sign of better times to come?

“Recent home buyers just entering the markets may be seeing price stabilization,” said Lawrence Yun, the usually optimistic chief economist for the National Association of Realtors.

He pointed out that in places like Las Vegas and Phoenix, drastically lower prices have led to an uptick in sales volume, a sign that conditions may stabilize.

But Patrick Newport, an economist with Global Insight, an economic forecasting firm, thinks there are more hard times ahead. He points out that seasonal variations may account for what appears to be a slowdown in the pace of the May decline.

“You can’t go by monthly numbers,” he said. “What I look at is the Census Bureau’s inventory of vacant homes on the market. That hasn’t budged much, although it dropped to 2.8% [of total homes for sale] in the second quarter from 2.9%.”

Historically, vacant homes have made up about 1.7% of housing inventory.

“What’s worrying me is that foreclosures are adding to inventory, and the inventory numbers tell you what to expect for the next couple of years,” says Newport. “They’re saying home prices will drop.”

And Yun expresses concern over mortgage rates, which have been on the rise. Higher rates can cancel out more affordable prices by increasing monthly mortgage payments.

The new housing rescue bill that just cleared Congress over the weekend may help, however. “The tax credit for first-time home buyers will offset the slight rise in mortgage rates,” he said.