The current paper utilizes a bifactor modeling approach to evaluate the structure of attention-deficit/hyperactivity disorder (ADHD) in adulthood and assess developmental continuity of ADHD structure between childhood and adulthood. The study compared traditional one-factor, two-factor, three-factor, and second-order factor models of ADHD with a bifactor model of ADHD. Developmental differences in ADHD structure were examined using an extension of the bifactor model: a two-group model comparing children and adults. Participants were 406 adults (49% male; 145 of 406 with ADHD), (18 to 37) years old, and 548 children (58% male; 302 of 548 with ADHD), 6 to 18 years old. A bifactor model of ADHD exhibited the best fit in adults and children compared to traditional models, suggesting continuity in the ADHD latent construct across development. However, significant differences in the factor loadings were evident between children and adults in the two-group bifactor model, suggesting changes in the relative importance of particular symptoms over time. Namely, hyperactivity symptoms appear to decline in importance relative to the ADHD phenotype between childhood and adulthood.