This randomized controlled experiment compared the efficacy of two response-to-intervention (RTI) models--typical RTI and dynamic RTI--and included 34 first-grade classrooms (n = 522 students) across 10 socioeconomically and culturally diverse schools. Typical RTI was designed to follow the two-stage RTI decision rules that wait to assess response to Tier 1 in many districts, whereas dynamic RTI provided Tier 2 or Tier 3 interventions immediately according to students' initial screening results. Interventions were identical across conditions except for when intervention began. Reading assessments included letter-sound, word, passage reading, and teacher-reported severity of reading difficulties. An intent-to-treat analysis based on multilevel modeling indicated an overall effect favoring the dynamic RTI condition (d = 0.36); growth curve analyses demonstrated that students in dynamic RTI showed an immediate score advantage and that effects accumulated across the year. Analyses of standard score outcomes confirmed that students in the dynamic condition who received Tier 2 and Tier 3 ended the study with significantly higher reading performances than students in the typical condition. Implications for RTI implementation practice and future research are discussed.