The purpose of this article is to present the findings of a quantitative study focused on primary mathematics teachers who participated in an intensive professional development program and then had leadership responsibility for the implementation of a new primary mathematics curriculum in their district. The study examines the effect of the professional development on teachers' and students' mathematical knowledge. Results indicate that effective implementation of a new curriculum can improve students' mathematical knowledge, but that the effect of the curriculum implementation on student mathematical knowledge is enhanced when teachers improve their mathematical knowledge for teaching and are simultaneously involved in the leadership activities of the curriculum implementation.