Therapeutic communities (TCs) have a strong record of maintaining high quality social climates in prison units. One possible reason for this is the system of mutual monitoring among TC residents, based on the assumption that peer affirmation of behavior in accord with TC norms, and peer correction of behavior contrary to TC norms, will lead to increased resident prosocial behavior. Laboratory experiments have demonstrated that such peer monitoring can lead to cooperation, but there has been no quantitative test of this hypothesis in an actual TC. In this article we test this assumption by using the affirmations that residents of three different TCs send as a measure of prosocial behavior following the reception of peer affirmations and corrections. At all three facilities residents send more affirmations following the reception of both affirmations and corrections, with this relationship being stronger and longer lasting after receiving affirmations. No other variable consistently predicts the number of affirmations that residents send to peers. These findings imply that mutual monitoring among TC residents can lead to increased levels of prosocial behavior within the facility, and that prosocial behavior in response to peer affirmations plays a key role. (Contains 4 tables.)