--- - |2+ Abstract This study examined longitudinal associations between externalizing symptoms and friendship quality among 116 adolescents and their same‐sex, same‐grade best friends (60.3% female). At the outset of the study, participants were in the sixth or seventh grades (M = 11.72 years). For two consecutive years, participants provided self‐reports of externalizing symptoms, and best friends provided perceptions of friendship quality (negativity and social support) and intimacy. Results indicated that greater externalizing symptoms predicted increases in friendship negativity for all dyads. Externalizing symptoms also predicted decreases in social support, but only among dyads reporting high levels of intimacy. The findings suggest that externalizing symptoms diminish friendship quality, particularly within intimate friendships. Highlights Greater externalizing symptoms may predict declines in friendship quality within intimate adolescent friendships. Path analyses found that externalizing symptoms predicted increases in negativity (for all friendships) and decreases in support (for intimate friendships). Results suggest a negative downward spiral where behaviour problems damage friendships, which then may become unstable. - Infant and Child Development, EarlyView.