The adoption of the Common Core State Standards for Mathematics (CCSSM, 2010) in 46 states has raised the mathematical proficiency bar for U.S. students. The advent of these new standards is at a time when the body of empirical evidence on the negative consequences of early and persistent difficulties in mathematics is clear, compelling, and growing. The purpose of this 4-year Goal-3 Efficacy Trial, funded by the Institute of Education Sciences (Clarke, Doabler, Smolkowski, & Fien, 2012), is to study the efficacy of ROOTS, a (Tier 2) kindergarten mathematics intervention designed to improve the mathematics achievement of students at risk for MD. This presentation focuses on data generated from the first year of the larger efficacy trial (2012-2013). The study used a randomized control trial (RCT) design (blocking on classrooms) to investigate the ROOTS intervention when implemented under rigorous experimental conditions. Year 1 of the ROOTS efficacy trial took place in 29 kindergarten classrooms from four school districts in Oregon with approximately 10 eligible students per classroom. The two primary aims of this presentation are: (1) present impact findings from the first year of the 4-year ROOTS efficacy trial; and (2) empirically examine the association between instructional intensity, manipulated via instructional group size, and student mathematics achievement. One table is appended.