The effect of friendship reciprocation and friend aggression on the stability of aggression across a 6-month period following the transition to secondary school was studied in a sample of 298 Grade 6 children from a predominately white, middle-class, Midwestern American community. The stability of aggression was generally high but it varied as a function of (1) the level of aggression of both individuals in the friendship and (2) whether the friendship was reciprocated. For children with high initial levels of aggression, those with unreciprocated aggressive friends were the most stable in their aggression. For children with low initial levels of aggression, most children remained stably low in aggression, with type of friendship and friend aggression having little effect on stability. Adolescents who were high in aggression at time 1 (T1) and had an aggressive friend (reciprocated or not) remained aggressive at time 2 (T2), but those who were aggressive at T1 and had nonaggressive friends actually displayed much lower levels of aggression at T2. The opposite did not occur for those adolescents low in aggression at T1. Those low in aggression with aggressive friends at T1 did not increase in aggression. These findings were discussed in light of current thinking about the effect of friendship on development. (Contains 1 table and 1 figure.)