We investigate the influence of intercollegiate athletic participation on grades using data from the U.S. Naval Academy. Athletic participation is an endogenous decision with respect to educational outcomes. To identify a causal effect, we develop an instrument via the Academy's random assignment of students into peer groups. Instrumental variables estimates reveal that sports participation reduces athletes' grades, on average, by nearly one-quarter of a letter grade. This finding has implications beyond college, as we also show that grades-not athletic participation-are most strongly associated with post-collegiate outcomes such as military tenure and promotion rates.