The physician developing a treatment plan for a particular patient often needs to know the life expectancy associated with the outcomes of therapeutic choices. Currently available methods for estimating life expectancy are cumbersome and of limited clinical use. We describe a simple approximation of life expectancy (the "DEALE") that is based on the assumption that survival follows a simple declining exponential function. In this approach, the reciprocal of the age-, sex-, and race-adjusted life expectancy is used to estimate the mortality rate of a healthy person. The life expectancy of a person who also has one or more diseases is obtained by adding disease-specific mortalities to the age-, sex-, and race-adjusted mortality rate and taking the reciprocal of that sum. In this paper we show that this approximation estimates life expectancy accurately for the great majority of clinical problems.