Math Teachers' Circles have been spreading since their emergence in 2006. These professional development programs, aimed primarily at middle-level mathematics teachers (grades 5-9), focus on developing teachers' mathematical problem solving skills, in line with the Common Core State Standards-Standards of Mathematical Practice. Yet, to date, anecdotal evidence and self-report data has been the primary measure of their effectiveness. This study examines the results of a three-site administration of the Learning Math for Teaching instrument, a multiple-choice instrument designed to measure aspects of Mathematical Knowledge for Teaching. Results indicate that Math Teachers' Circles are impacting teachers' performance on the Number Concept and Operation subsection, leading to implications for future research.