Lesson study, especially when conducted with support from targeted resources, is often identified as a potentially effective means to support teacher learning. In this study, 80 school-based teams of educators, representing 80 classrooms of third- or fourth-grade students in the US, were randomly assigned to one of four conditions: lesson study supported by fractions resources, lesson study only, fractions resources only, or a comparison group. Fractions instruction was improved in the two conditions that received the fractions resources. Average student performance on a fractions test did not increase relative to the comparison group in any of the three intervention conditions. However, moderation analyses suggest that lesson study, both with and without fractions resources, may have improved student performance in one state, which represented half of the sample.