Anxiety sensitivity is associated with both anxiety and depression and has been shown to be heritable. Little, however, is known about the role of genetic influence on continuity and change of symptoms over time. The authors' aim was to examine the stability of anxiety sensitivity during adolescence. By using a genetically sensitive design, the authors were also able to investigate the extent to which genetic and environmental factors influence anxiety sensitivity over time. Self-reports of anxiety sensitivity were obtained for over 1,300 twin and sibling pairs at 3 time points. Data were analyzed using multivariate genetic models. Anxiety sensitivity was moderately heritable at all time points with substantial nonshared environmental contributions. Time 1 genetic factors accounted for continuity of symptoms at Times 2 and 3. New genetic factors at Time 2 also influenced Time 3 symptoms. New nonshared environmental influences emerged at each time point. Analysis of a latent factor of trait anxiety sensitivity revealed some stable nonshared environmental influences. Genetic effects were generally stable over time, with new genetic influences emerging in late adolescence. Environmental influences on anxiety sensitivity were, on the whole, more time specific; however, some stable environmental influences were also found. (Contains 2 figures and 4 tables.)