Spikes in symptom severity are noted for adolescents with attention deficit/hyperactivity disorder (ADHD) at the transitions to middle and high school that are attributed to developmental maladjustment. This study evaluated the effectiveness of high-intensity (HI; 412 hr, $4,373 per participant) versus low-intensity (LI; 24 hr, $97 per participant) skills-based summer intervention delivered to adolescents with ADHD by local school district staff. Participants were 325 ethnically diverse rising sixth and ninth graders with ADHD randomized to HI versus LI (n = 218) or recruited into an untreated comparison group (n = 107). Group × Time 1-year outcome trajectories were compared using linear mixed models. Both interventions possessed high fidelity and were viewed by families as enjoyable and beneficial. Youth attendance was higher for HI (~80%) versus LI (~45%). Parent training attendance was uniform across groups (~50%). Parent and student attendance did not impact trajectories. Primary benefits of HI over LI were to note taking (d = .50), parent contingency management (d = .43), and parent-rated ADHD symptoms (d = .40-.46; ninth grade only). Secondary analyses suggested that HI may produce additional benefits compared to no treatment for home organization skills (HI vs. untreated d = .54), parent-teen conflict (HI vs. untreated d = .39), and grade point average (HI vs. untreated d = .47, ninth grade only). Summer HI treatment was superior to LI in engagement and uptake of certain skills. However, the extent to which these medium benefits on a limited number of outcomes justify high costs compared to LI remains an open question. Delivering treatment during the summer instead of school year may limit generalizability.