Empirical studies investigating supplemental reading interventions for students with or at risk for reading disabilities in the early elementary grades have demonstrated a range of effect sizes. Identifying the findings from high-quality research can provide greater certainty of findings related to the effectiveness of supplemental reading interventions. This meta-analysis investigated how four variables of study quality (study design, statistical treatment, Type I error, and fidelity of implementation) were related to effect sizes from standardized measures of foundational reading skills and language and comprehension. The results from 88 studies indicated that year of publication was a significant predictor of effect sizes for both standardized measures of foundational reading skills and language and comprehension, with more recent studies demonstrating smaller effect sizes. Results also demonstrated that with the exception of research design predicting effect sizes on foundational reading skills measures, study quality was not related to the effects of supplemental reading interventions. Implications for research and practice are discussed.