This multiyear scale-up effectiveness study of Open Court Reading (OCR) involved approximately 4,500 students and more than 1,000 teachers per year in Grades K-5 from 49 elementary schools in seven districts across the country. Using a school-level cluster randomized trial design, we assessed the implementation and effectiveness of Open Court Reading over two years. Implementation study results demonstrated adequate to high levels of fidelity across the treatment schools. Intent-to-treat analyses revealed no statistically significant main effects on students' reading performance in Year 1 and a small negative effect (d = -0.09) in Year 2. There were positive impacts for particular subgroups, including kindergarten (d = 0.12) and Hispanic (d = 0.10) students in the first year. However, there were negative impacts for first grade (d = -0.13), females (d = -0.11), students who were not eligible for free or reduced-price lunch (d = -0.19), and non-English language learners (d = -0.10) in the second year of the study. Thus, relative to the "business-as-usual" reading curricula, no positive overall impacts of OCR and mixed impacts for student subgroups were found.