Purpose This article includes results from a multistate randomized controlled trial designed to investigate the impacts of a language-focused classroom intervention on primary grade students' proximal language skills and distal reading comprehension skills. Method The sample included 938 children from 160 classrooms in 4 geographic regions in the United States; each classroom was randomly assigned to 1 of 2 experimental conditions (2 variations of a language-focused intervention) or business-as-usual control. For this study, the 2 experimental conditions were collapsed, as they represented minor differences in the language-focused intervention. All children completed assessments at multiple time points during the academic year. Proximal measures (curriculum-aligned measures of vocabulary, comprehension monitoring, and understanding narrative and expository text) were administered throughout the school year. Distal measures of reading comprehension were administered at the beginning and the end of the school year. Results Multilevel multivariate regression was conducted with results showing that students receiving the language-focused intervention significantly outperformed those in the control group in comprehension monitoring and vocabulary, with effect sizes ranging from 0.55 to 1.98. A small effect in understanding text (narrative) was found in 3rd grade only. Multilevel path analyses were then conducted to examine if the intervention had a positive impact on reading comprehension through the influence of proximal language outcomes. In all 3 grades, instruction impacted reading comprehension via the mediation of vocabulary, with sizable effects (1.89-2.26); no other indirect pathways were significant. Conclusions This study provides evidence that a language-focused intervention can positively impact students' performance on language measures that are closely aligned with the intervention, with indirect, large effects on distal reading comprehension measures. Theoretically, this study provides causally interpretable support for the language bases of reading comprehension.