Negative amortization, or "deferred interest," describes loans that have payment adjustment caps in addition to interest rate adjustment caps. Negative amortization loans calculate two interest rates. The first is called the payment rate the second is the actual interest rate. The payment rate is typically capped at 7.5% of the previous payment. The true interest rate is calculated as simply the index plus the margin without periodic caps. When the interest rate resets to a higher rate with a negative amortization Adjustable Rate Mortgage (ARM), the mortgage payment doesn't change. Instead, the additional interest expense is added to the loan balance.

Borrowers are given a choice of which rate to pay, which is why negative amortization loans are also referred to as "payment option" loans and option ARMs. Cost of Funds Index (COFI), Cost of Savings Index (COSI), and Monthly Treasury Average (MTA or MAT) are all examples of Alt-A negative amortization loans. The Mortgage Bankers Association of America (MBA) says alt-A loans' share rose from 8% to 11%. Why? Because of the flexibility these loans offer, not to mention affordability for a home purchase loan or if you want to cash out on your home equity with a mortgage refinance.

Some lenders and public officials estimate that 60 percent of today's mortgages have a negative-amortization feature. A survey of 239 of the country's largest savings and loan associations by the Federal Home Loan Bank Board in January found that 33 percent allowed it. Another affordable loan option is the interest only loan. With an interest-only loan, you pay only the interest on the mortgage in monthly payments for a fixed term. After the end of that term, usually five to seven years, you must refinance, pay the balance in a lump sum, or start paying off the principal, which increases your monthly payments substantially. Like negative amortization loans, interest-only loans are option ARMs because borrowers have the option of paying only the interest or paying principal and interest.

Negative amortization and interest-only loans can be useful if you are primarily concerned with cash flow instead of building equity. If you only pay the payment rate, the overall monthly mortgage payment might be lower than a typical 30-year, amortization loan. You might want to consider a negative amortization or interest only mortgage if you're a short-term borrower who plans to refinance or sell the home within a period of a few years or if you have unsteady sources of income or too little documented income to qualify for a traditional loan.