Early mathematics achievement is a strong predictor of later mathematics achievement, and student competence in fractions and division may be the strongest predictor of success in secondary-level algebra. We used a multisite cluster-randomized trial to estimate the impact of a mathematics teacher professional development (PD) program on student mathematics achievement. Designed for grades 3-5 teachers, the program consisted of 9 days of teacher workshops and focused on number, operations, and algebraic thinking with emphasis on fractions. The randomized sample included 149 teachers and their students, representing 31 schools in 9 public school districts in Florida. With a low per student cost, the Cognitively Guided Instruction 3-5 PD program had a positive impact on student performance on the Elementary Mathematics Student Assessment (g = 0.19, p = 0.004). The magnitude of the impact on student mathematics achievement suggests that these results may also have practical importance.