Response to intervention (RTI) holds great promise for the early identification and prevention of reading disabilities. The success of RTI rests in part on the accuracy of universal screening tools used within this framework. Despite advancements, screening instruments designed to identify children at risk for reading disabilities continue to have limited predictive validity. In this study, the authors examined a common screening instrument for the presence of floor effects and investigated the impact that these effects have on the predictive validity of the instrument. Longitudinal data (kindergarten to third grade) from a large cohort of children were used. These data included children's performance on five measures from the "Dynamic Indicators of Basic Early Literacy Skills" (DIBELS) and two reading achievement outcome measures. The results showed that DIBELS measures were characterized by floor effects in their initial administrations and that these effects reduced the predictive validity of the measures. The implications of these findings for early identification are discussed. (Contains 2 tables, 2 figures, and 5 notes.)