This study examines the impact of a nationally disseminated professional development program, Developing Mathematical Ideas (DMI), on teachers' specialized knowledge for teaching mathematics and illustrates how such research could be conducted. Participants completing 2 DMI modules were compared with similar colleagues who had not taken DMI. Teacher knowledge was measured with multiple-choice items developed by the Learning Mathematics for Teaching project and open-ended items based on problems initially developed by DMI experts. After controlling for pretest scores, a hierarchical linear model identified statistically significant differences: The DMI group outperformed the comparison group on both assessments. Gains in teachers' scores on the more closely aligned measure were related to the degree of facilitator experience with DMI. This study adds to our understanding of the ways in which professional development program features, facilitators, and issues of scale interact in the development of teachers' mathematical knowledge for teaching. Study limitations and challenges are discussed. (Contains 12 footnotes, 6 tables, and 1 figure.)