The U.S. Treasury has announced that it will issue 30-year bonds for the first time in five years, and many investors may be asking, "What's in it for me?"

As an investment, the long bond offers little, says Greg McBride, a senior financial analyst with Bankrate.com in North Palm Beach, Fla.

"If you have a 30-year investment horizon, you should be pursuing higher returns than you can get on a Treasury," says McBride. He expects the interest on the new 30-year bonds to be 4.5%. That's about the same as what a 10-year Treasury pays now.

If your investment horizon is less than 30 years, you face significant risk with 30-year bonds, which are more sensitive to interest rate changes than short-term bonds. "If you have to sell in an environment of rising interest rates, you could sell it substantially below what you paid for it," says McBride.

But if pension funds and insurance companies start snapping up the 30-year bond in place of 10-year Treasury notes, that could spell trouble for mortgage rates. Why?

Banks often package their mortgages in investment pools and sell them as bonds called mortgage-backed securities. They trade like 10-year bonds because the underlying securities — the mortgages — are typically paid off in no more than 10 years as people refinance their homes or move.

If more institutional investors begin buying 30-year bonds, then the price of 10-year bonds may fall, pushing up the yield, which moves in the opposite direction. A rising yield on the 10-year note could increase 30-year mortgage rates by 0.25 percentage points or more.

What's more, McBride says there's evidence that this dynamic has played out before, only in reverse. When the Treasury discontinued the 30-year bond in 2001, demand for the 10-year bond soared, sending the yield — and mortgage rates — much lower.

McBride says mortgage rates already face upward pressure because of worries about inflation and the strength of the dollar. Taken together, he thinks the time is right to lock in a long-term fixed rate on a mortgage.