The purpose of this longitudinal study was to investigate the relationships between mathematics teacher preparation and graduates' analyses of classroom teaching. Fifty-three graduates from an elementary teacher preparation program completed 4 video-based, analysis-of-teaching tasks in the semester before graduation and then in each of the 3 summers following graduation. Participants performed significantly better on the 3 tasks focused on mathematics topics studied in the program than on the task focused on a mathematics topic not studied in the program. After checking several alternative hypotheses, we conclude that a likely explanation for the performance differences is the mathematical knowledge for teaching that participants developed as freshmen in the preparation program.