This study examined math achievement of elementary school students when Math Out of the Box (MTB), an inquiry-based math program, was used to supplement curriculum. The sample consisted of 767 New Jersey students in the third, fourth, and fifth grades, with approximately one third using MTB. Math achievement was measured by an assessment developed by ETS and by New Jersey's standardized test of math proficiency (NJ ASK). On the ETS assessment, a small, statistically significant difference was found in each of the three grades between students who used MTB and those who did not. On NJ ASK, a small, statistically significant difference was found in the third grade only. While these findings are an important step in establishing the efficacy of MTB, selection bias may weaken the causal inferences drawn. One appendix is included containing tables with results. (Contains 15 tables.)