We use data from one of the few states where information on curriculum adoptions is available--Indiana--to empirically evaluate differences in performance across three elementary-mathematics curricula. The three curricula that we evaluate were popular nationally during the time of our study, and two of the three remain popular today. We find large differences in effectiveness between the curricula, most notably between the two that held the largest market shares in Indiana. Both are best characterized as traditional in pedagogy. We also show that the publisher of the least-effective curriculum did not lose market share in Indiana in the following adoption cycle; one explanation is that educational decision makers lack information about differences in curricular effectiveness. (Contains 6 tables and 35 notes.)