If you're in the market for a mortgage, don't wait.

Mortgage rates have shot up almost a full percentage point in the last 12 months, hovering at their highest levels in nearly four years, and are expected to go only higher.

The average rate for a 30-year, fixed-rate loan ticked up to 6.64 percent this week, according to the interest rate tracker Bankrate.com. That's up from 6.27 percent at the beginning of the year and 5.85 percent this time last year.

On a $150,000 mortgage, the increase translates into an extra $77 per month.

For a 15-year mortgage, the average rate rose to 6.27 percent from 5.42 percent a year ago, Bankrate.com said, while the rate for a one-year adjustable hit 5.87 percent, up from 4.59 percent -- the highest level in nearly five years.

Despite the run-up, don't fret too much. By historical standards, rates still look pretty good. The average rate on a 30-year loan isn't all that far from the record low of 5.3 percent set in June 2003.

The biggest shock has been for holders of adjustable-rate mortgages. Adjustable-rate mortgages, or ARMs, typically have a lower initial interest rate than fixed mortgages, but after a certain period of time the rate varies according to an index.

Rates on ARMs have been rising more quickly than fixed-rate loans, in tandem with the Federal Reserve's series of increases in short-term rates aimed at battling inflationary pressures.

"If you have an adjustable-rate mortgage poised for an increase any time in the next two years, you are standing on the train tracks with the bright lights bearing down on you," said Greg McBride, financial analyst at Bankrate.com.

"Now is the time to get out of harms way," he said, by refinancing into a fixed-rate mortgage while rates remain relatively low.

Whether refinancing makes sense largely depends on whether you'll be in your home long enough to recoup the closing costs. Generally, if you plan to stay put for more than three years, refinancing is worth looking into, Mr. McBride said.

Typically, ARMs don't carry prepayment penalties. But such penalties could be an additional cost for homeowners with poor credit who are holding so-called "subprime" loans that carry higher-than-market rates.

At the same time that the Fed has been consistently raising short-term rates since June 2004, long-term rates that serve as the benchmark for fixed-rate mortgages have remained stubbornly low and only in the past year have begun to inch up.

As a result, the traditional spread between rates on adjustable and fixed-rate loans has narrowed dramatically, making fixed-rate mortgages a better deal for most home buyers, Mr. McBride said.

Nevertheless, the upturn in long-term rates has accelerated in recent months, aided by market pressure for a more normal spread between short- and long-term rates and stoked by concerns about higher energy costs and overall inflation.

Economists generally expect the average rate on a 30-year mortgage to rise modestly in the coming months, edging up to the 7 percent range by year-end.

"Fixed-rate mortgages will continue trending higher as the year progresses, but are likely to remain very attractive relative to adjustable rates as well as the historical average," Mr. McBride said.

Mr. McBride isn't a fan of interest-only mortgages, which allow borrowers to defer principal payments for five years or more and which have been gaining in popularity in recent years, especially in areas with skyrocketing home prices.

Interest-only mortgages accounted for an estimated 27 percent of all home loans last year, up from 23 percent in 2004 and just 1 percent in 2000, according to LoanPerformance LLC, a San Francisco-based mortgage data firm. In Pittsburgh, interest-only mortgages made up an estimated 7 percent of the market in 2005, up from under 1 percent in 2000.

With interest-only loans, payments in the initial years are artificially low, setting up borrowers for payment shock when they must start paying off the principal, Mr. McBride said.

At today's relatively low rates, home buyers who can't afford the monthly payment on a 30-year mortgage are buying too much home, he said.

"If you have to resort to an interest-only [loan] or ARM to squeeze into that house, you are setting yourself up for trouble."

---

Correction/Clarification: (Published April 28, 2006)The mortgage chart originally included with this article in April 27, 2006 editions was incorrect. The correct chart is now displayed in this article.