ABSTRACT With the increasing use of adjustable‐rate mortgages for asset/liability management, there exists the need to properly evaluate their price sensitivity to interest rate changes. This paper provides a foundation by deriving the duration of an adjustable‐rate mortgage. The properties of this duration are unique and have some important differences from those of fixed‐rate securities. One important characteristic of an adjustable‐rate mortgage concerns the index used to adjust the mortgage rate. It was found that the index tended to be more important than the adjustment frequency in determining the duration of an adjustable‐rate mortgage.