This paper examines the impact of an intensive professional development on practicing teachers' mathematics discussion-leading practice. A tool for examining specific discussion-leading moves was used to measure change in teachers' practice as observed through submitted video recordings of mathematics discussions. Participants included 33 teachers from three school districts who submitted a total of 193 videos across the study. The findings reveal that the professional development had differential impact on participating groups. We explore group attributes that could contribute to these differential outcomes. [For the complete proceedings, see ED630210.]