The decline in stature with age among adults is well documented. Although part of this represents a birth cohort effect, actual height declines among older individuals are known to contribute to the effect. In this study we used longitudinal changes in the heights of adults in a general population sample to determine the rate of decline in height over time in individuals of different ages. This allowed an estimation of the age at which decline in height begins, a value close to age 40 in both sexes. It also allowed derivation of equations from which the maximum height of subjects can be estimated on the basis of their sex, current height, and age. These equations should prove useful when examining the effect of aging per se on physiological measurements that are height dependent. The data also allow one to compare the magnitude of the effect of year of birth with that of the actual decline in height seen among the elderly. We estimate that approximately 60% of the smaller stature of older male subjects and 45% of the smaller stature of older female subjects is a birth cohort effect deriving from the secular trend toward greater stature; the remainder is a result of an actual decrement in height after the age of 40.