The current study prospectively examined trajectories of change in symptoms of irritability, hyperactivity, and social withdrawal, as well as predictors of such behaviors, for ages 9–18 years for youths with autism spectrum disorder and a comparison group with nonspectrum developmental delays. Children with more severe core features of autism had consistently higher irritability and hyperactivity scores over time than those with broader autism spectrum disorder and nonspectrum delays. Across all diagnoses, behaviors related to hyperactivity showed the greatest improvement. Social withdrawal worsened with age for a substantial proportion of youths with autism spectrum disorder but not for the nonspectrum comparison group. Compared with youths without autism spectrum disorder, children with the disorder showed greater heterogeneity in trajectories for maladaptive behaviors.