Low-achieving students in many school districts are retained in a grade in order to allow them to gain the academic or social skills that teachers believe are necessary to succeed academically. This practice is highly controversial, with many researchers claiming that it leads to higher dropout rates although selection issues have complicated previous analyses. In this paper, we use a regression discontinuity design to examine the impact of grade retention on high school completion. We find that grade retention leads to a modest increase in the probability of dropping out for older students, but has no significant effect on younger students.