Research Findings: Big Math for Little Kids (BMLK) is a mathematics curriculum designed for 4- and 5-year-old children. In this study, the curriculum was evaluated for effectiveness over two years, using a cluster-randomized controlled study. Over 750 children participated in the study and experienced either the BMLK curriculum or business-as-usual instruction. Students' mathematics knowledge was assessed using the Early Childhood Longitudinal Study-Birth Cohort (ECLS-B) Direct Mathematics Assessment, an independent outcome measure not tied to the curriculum materials. The BMLK children significantly outperformed the business-as-usual control group, a difference that represents the equivalent of 1.6 months of additional instruction, with a medium effect size (Cohen's d=0.40). BMLK children also showed indications of improved mathematical language on piloted language tasks. Policy or Practice: These results suggest that the inclusion of thoughtful, developmentally appropriate mathematics curriculum can positively impact young students' achievement. (author abstract)