Mathematics teacher professional-development (PD) programs based on Cognitively Guided Instruction (CGI) have been developed and implemented with tens of thousands of elementary mathematics teachers worldwide since the 1980s. Starting with a focus on addition and subtraction on whole numbers in the first CGI PD program, the content of these courses has expanded over time. In this study, we used a multisite cluster-randomized trial to estimate the impact of the first year of a three-year CGI teacher professional-development program. This CGI PD program designed for grades three, four, and five teachers. The content of the program focused on fractions concepts and operations on fractions, multiplication and division on whole numbers, and algebraic thinking. The randomized sample included 149 teachers and their students, representing 32 schools in nine public school districts in Florida. Student outcomes were measured by the Elementary Mathematics Student Assessment (EMSA). The CGI 3-5 PD program was found to significantly impact student performance on the EMSA test (p = 0.007) in the first year of the three-year program. The point estimate of the main effect of the CGI program on student achievement (g = 0.18) was greater than many of the most effective teacher professional-development programs that have been subjected to rigorous evaluation.