This study addresses the effectiveness of a nationally used core reading program that reflects the research-based practices recommended by the National Reading Panel. This and other similar programs are increasingly used to prevent reading difficulties and ensure that all children are reading at or above grade level by the end of third grade. The "Open Court Reading" (OCR) program, published by SRA/McGraw-Hill and widely used since the 1960s, offers a phonics-based K-6 curriculum that shows promise for preventing reading difficulties. According to market research, OCR is among the top reading series (Educational Market Research, 2002). To date, a total of 2,917 districts and over 8,600 schools have adopted the OCR program across all 50 states and Washington DC (SRA/McGraw-Hill, 2008). Findings from independent non-experimental evaluations suggest that, in comparison to other reading curricula, OCR is associated with significantly better reading outcomes and may be particularly effective with low-performing students (Edsource, 2006; McRae, 2002; Skindrud & Gersten, 2006). Despite the program's widespread use and promising research findings, OCR has not been evaluated rigorously on a large scale as part of an objective, third-party evaluation. In this study, an independent research team evaluated the effectiveness of the OCR program in a sample of 49 elementary schools (kindergarten through 5th grade) in 7 districts across the country, across diverse school populations and conditions, and with no more support than schools would have access to if they had selected OCR as their early reading curriculum apart from participation in a research project. Tables and figures are appended.