Two measures of stability based on longitudinal growth data are described. One, termed a growth constancy index, is based on the variation of each individual's (standardized) measurement about his or her average. The other, called a growth separation index, measures the extent to which individual growth curves cross each other. The uses of these indices are described and formulae for calculating them and their standard errors are given. They are applied to an example involving group comparisons of height, weight and skinfold data.