It's no surprise -- the longer you own your home, the greater chance you have of realizing a return on the investment.

But then, there's the other side of the coin.

In recent years, the risk of your home's value declining has also grown with time as has the number of metropolitan areas facing a greater risk of home value declines.

Walnut Creek, CA-based PMI Mortgage Insurance Co.'s Spring Economic and Real Estate Trends (ERET) report says from 1986 to 2005, if you owned a home in one of the 50 largest metropolitan statistical areas (MSAs) your home has enjoyed a sizable increase in value.

"What we found was that across the nation's 50 largest MSAs, owning a home for 10 years or more resulted in a positive return in 100 percent of the cases," explained Mark Milner, chief risk officer at PMI Mortgage Insurance Co.

"This dropped to 95 percent with a seven-year ownership term and to 92 percent with a five-year ownership term -- still a pretty impressive rate. Home ownership clearly can be an important strategy for building wealth over the long term," Milner said.

That doesn't mean time is completely on your side.

Forty-eight of the nation's 50 largest MSAs face a greater risk of declining home prices this quarter than they did the last quarter, according to PMI's Market Risk Index, which reveals, as time goes by, the risk of home value plunge goes up.

Risk Index scores increased for all of the top 50 MSAs except Chicago, IL, whose score decreased one point. New Orleans was not scored this quarter because the index is not designed to measure the impact of catastrophic events, including Gulf Coast storms.

PMI said 14 of the top 50 MSAs now have risk scores above 500, meaning they face a 50 percent or greater risk of home price declines in the next two years, up from only 11 MSAs last quarter. The average score has increased from 261 last quarter to 287. The biggest change was in Minneapolis, MN, which gained 90 points, taking it to a score of 350 and up two spots in the ranking to No. 19.

Seven of the 10 MSAs with the highest risk scores were in California, the other three were clustered in the Northeast. The top five scores came from the San Diego, CA area MSA (598); Santa Ana, CA area MSA (589); Boston, MA area MSA (588); Nassau, NY area MSA (586); and Riverside, CA area MSA (579).

The index scale ranges from one to 1,000 and translates to a percentage. For example, a score of 100 indicates a 10 percent chance of a decline in home prices over the next two years. A higher score indicates a higher likelihood of future home price declines. An increase in a risk index score of 100 percent, say, from 100 to 200, indicates that the risk of home price decline has doubled.

There's more bad news.

PMI's Affordability Index reveals affordability decreased in all 50 of the nation's largest MSAs in the fourth quarter of 2005. Eight MSAs have Affordability Index scores below 70, which PMI considers a threshold for vulnerability to an economic shock. Only two MSAs last quarter had scores below 70.

The five MSAs with the lowest affordability indexes were the Fort Lauderdale, FL area MSA (62.55); Riverside, CA area MSA (63.05); Los Angeles, CA area MSA (64.16); Santa Ana, CA area MSA (65.08) and Oakland, CA area MSA (67.08).

The Affordability Index, a component of the Market Risk Index, considers income, home price appreciation and financing costs on a linear scale with a baseline of 100 in 1995. For example, an Affordability Index score of 85 means that the median home in that area is 15 percent less affordable than it was in 1995.

PMI says the lower affordability index numbers reflect the increase in interest rates in the fourth quarter of 2005, as well as home price increases out pacing income increases.

"The most significant change we saw this quarter was in affordability," Milner said.

"With continued double-digit appreciation in many areas coinciding with higher interest rates, affordability is becoming more of a challenge for American home buyers," he added.

Twenty-six of the 50 largest MSAs saw double-digit appreciation, led by Phoenix, AZ at 33.4 percent; Orlando, FL, 27.64 percent and Fort Lauderdale, FL, 25.32 percent.

The good news is that the rate of home price appreciation is decelerating -- home prices are still rising, but more slowly.

PMI said appreciation slowed in 21 of the 50 largest MSAs.

The biggest changes were in Las Vegas, NV, where appreciation slowed by 18 percentage points, in San Diego, CA, where it slowed by 13 percentage points and in Orlando, FL, where is slowed by more than 12 percent.