The issue of whether prison sentences decrease or increase crime is far from settled, largely due to the difficulty associated with isolating the potential mechanisms that might operate in the prison environment. Deterrence and rehabilitation theories argue that the prison experience should reduce crime, whereas oft-posited, yet sparsely tested, theories argue that prisons are “schools of crime” in which peer influence breeds future offending. With Pennsylvania data for the members of a first-time adult release cohort and their cellmates, we estimate a dose-response effect of peers on reoffending, thus shedding light on an important mechanism that contributes to the effect of imprisonment on those who experience it.