The research on professional development (PD) consists mostly of studies utilizing cross-sectional data. We examined effects of change in school-level PD on change in teachers' practice longitudinally. Using survey reports from 1,722 teachers in 31 schools implementing a popular comprehensive school reform (America's Choice), we found that although schools were implementing the same reform model, they varied in their implementation of PD. Teachers successfully changed their practice toward reform-aligned instructional goals only in schools demonstrating high growth in providing reform-aligned PD (both in the content and process of their PD). Our findings have implications for understanding interventions in schools and for thinking about the rigor of PD effects on teaching.