Numerous studies have examined how grade retention can impede the academic success of those retained students. One uncharted line of research is the spillover effect that retained students may exert on their classmates. The purpose of this study is to evaluate this relationship for students in urban elementary school classrooms. To do so, this study analyzes a longitudinal data set comprising entire populations of five elementary school cohorts within the School District of Philadelphia. Because individual student records can be linked to teacher and classroom data as well as to school, grade, and year identifiers, this study employs a series of multilevel fixed-effects models to address estimation issues regarding omitted variable bias. All results indicate that the effects of having a greater number of grade-retained peers are detrimental to the standardized achievement outcomes of nonretained classmates. Data-driven policy implications are discussed. (Contains 7 tables and 10 notes.)