This study investigates, through an experimental design, the effectiveness of a professional development program on teacher knowledge and practices and on student learning. The program consisted of a series of video-based modules designed to respond to needs of U.S. teachers, as highlighted by findings from the 1999 Third International Mathematics and Science Video Study. Sixty-four 6th-grade teachers from five low-performing inner-city schools participated in the study and were randomly assigned to treatment and control groups. Measures included fidelity of implementation, teacher knowledge and practice, and student mathematics learning. The program did not impact significantly teacher knowledge or practices as measured in the study. An effect was found on mathematics learning for students whose teachers reached a certain level of mathematics content knowledge. Discussion of findings includes lessons learned about conducting and studying professional development, particularly in low-performing schools. (Contains 1 figure, 4 footnotes, and 4 tables.)