Objective: Significant anxiety often occurs in the presence of ADHD symptoms; however, the reasons are not well understood. We aimed to establish whether the relations between ADHD symptons and anxiety are bidirectional or unidirectional. Method: Weexamined the developmental relations between ADHD and anxiety symptoms across adolescence (ages 13, 15, and 17) in a community-ascertained, normative longitudinal sample of 1,483 youth (52% male). We used an autoregressive latent trajectory model with structured residuals (ALT-SR) to examine within-person developmental relations between ADHD and anxiety symptoms to determine whether it is ADHD symptoms that lead to anxiety symptoms and/or the reverse. Results: Results suggested that there are reciprocal within-person developmental relations between ADHD and anxiety symptoms. Conclusions: Our findings support the recommendation that targeting ADHD symptoms can be fruitful for addressing anxiety symptoms; however, they suggest that targeting anxiety symptoms may also benefit ADHD symptoms. Results also underline the importance of careful assessment for underlying ADHD symptoms among adolescents presenting with anxiety.