The authors describe results from a study of a middle school mathematics formative assessment strategy. They employed a randomized, controlled design to address the following question: Does using our strategy improve student performance on assessments of key mathematical ideas relative to a comparison group? Eighty-five teachers and 4,091 students were included. Students took a pretest and a transfer measure at the end of the year. Treatment students completed formative assessments. Treatment teachers had exposure to professional development and instructional resources. Results indicated students with higher pretest scores benefited more from the treatment compared to students with lower pretest scores. In addition treatment students significantly outperformed control students on distributive property items. This effect was larger as pretest scores increased. Results, limitations, and future directions are discussed. (Contains 7 tables and 5 figures.)