This study examined the effect of close friendship intensity as a potential amplifier of an adolescent's preexisting tendencies toward depressive and aggressive symptoms. A diverse community sample of 170 adolescents and their closest friends was assessed via multiple methods, and adolescents were followed from age 16 to 17. Results supported the hypothesized effect, with more intense close friendships interacting with higher baseline levels of behavioral symptoms to predict greater relative increases in symptoms over time. Effects were observed for both depressive and aggressive symptoms, and appeared with respect to multiple observational measures of friendship intensity. Findings are interpreted as suggesting that seemingly disparate phenomena (e.g., co‐rumination for depression and deviancy‐training for aggression) may both be dependent upon the intensity of the adolescent's social connections.