This study addressed the effects of multiyear, response-based, tiered intervention for struggling readers in grades 6-8. A sample of 768 sixth-grade students with reading difficulties was randomized to a response-based, tiered-intervention condition or "business as usual," and initial treatment status was maintained over the three-year study. To estimate the effect of treatment and to address questions about the acceleration of learning, a multiple-indicator, multilevel growth model was fit, representing the likely trajectories of the group of students originally randomized (in the fall of sixth grade) to treatment. Three-year trajectories were fit, with the results representing likely multiyear trends for the three groups. Treatment students, on average, outperformed business-as-usual students. The effect size based on the multiple-indicator, multilevel model was 0.26. Treated students also outperformed the group of typical readers when achievement was characterized in terms of slope over time. However, a sizable gap remained between treated and typical students in the spring of eighth grade. (Contains 2 tables, 2 figures, and 1 note.)