The long-term effects of Professional Development (PD) on educators' ability to affect student performance remain virtually unstudied. This quasi-experimental, longitudinal study compared student achievement scores from 25 states and 78 school districts over multiple years for schools whose teachers used an online, on-demand professional development offering. The objective was to evaluate to what degree student scores might improve, decline, or plateau with participation in professional development over multiple years. Results showed significant gains in student performance across the number of years of participation, with significant continuation in growth across years. Findings validate a significant predictive relationship between the number of years educators participated in PD and improvements quantified in student performance. Inferences of findings to other PD approaches or offerings, or other student or educator populations or settings are discussed.