With an adjustable-rate mortgage (ARM) on which the borrower is making the fully amortizing payment, extra payments do change the monthly payment, but not until the next rate adjustment. At that point, the payment is recalculated and the new payment will reflect all prior reductions in the balance.

Assume the $100,000 6 percent loan is a one-year ARM, and that an extra payment of $90,000 is made in month 2. The payment would remain at $599.56 through month 12, but (assuming the rate stayed at 6 percent) the payment would drop to $13.81 in month 13.

On ARMs with longer initial rate periods, the drop in payment following an extra payment would be further delayed. On the popular 5-year ARM, for example, the payment wouldn't drop until month 61.

If a loan is interest-only, the payment should decline in the month following an extra payment, whether the loan is fixed-rate or adjustable-rate. The interest only payment on the $100,000 loan at 6 percent is $500. Following the payment of $90,000 in month 2, the interest-only payment should drop to $50 in month 3.

From the mail I have received on this topic, however, I get the distinct impression that not all lenders have their servicing systems geared to do this properly. This is not surprising, given the haste with which many lenders have incorporated interest-only into their program offerings. Even if the system calculated the new interest-only payment correctly, they need to communicate the new interest-only payment to the borrower. I have not done a comprehensive survey, but I do know that some lenders are not doing this.

In most cases, lenders who do not change the payment immediately will change it on the anniversary month, as specified in the note. Until that date, the payment will remain unchanged, but since the interest due is lower, a part of the payment will be credited to principal.

If the loan in my example is of this type, the interest due in month 3 will drop to $50, but the borrower will continue to pay $500 until month 13, which is the anniversary month. $450 will be applied to principal in month 3. In each subsequent month 4-12, the interest portion will drop a little and the principal portion will rise, until month 13, when the borrower will once again be able to pay interest only.

There are some interest-only loans on which the interest-only payment in month 1 continues until the end of the interest-only period – five or 10 years.

If it is an ARM, the payment will adjust when the rate adjusts, but if it is fixed-rate, the payment won't change for five or 10 years.

If you are contemplating an interest-only loan and find immediate payment adjustments in response to extra payments a highly desirable feature, ask about it. Don't expect the subject to be volunteered by the loan officer or mortgage broker. They are not involved in loan servicing and the chances are that they don't know the answer and will have to ask. Make sure they do.