This study reports the results of a cluster RCT evaluating the impact of Enhanced Core Reading Instruction on reading achievement of grade 1 at-risk readers. Forty-four elementary schools, blocked by district, were randomly assigned to condition. In both conditions, at-risk readers received 90 minutes of whole-group instruction (Tier 1) plus an additional 30 minutes of daily, small-group intervention (Tier 2). In the treatment condition, Tier 1 instruction included enhancements to the core program and Tier 2 intervention was highly aligned with the core program. In the comparison condition, Tier 1 instruction used the same core program as treatment schools in the district and Tier 2 intervention followed standard district protocol. Significant treatment effects were found on measures of phonemic decoding and oral reading fluency from fall to winter and word reading from fall to spring. Student- and classroom-level variables predicted student response to instruction differentially by condition.