This research addresses a new construct, co-rumination. Co-rumination refers to extensively discussing and revisiting problems, speculating about problems, and focusing on negative feelings. Friendship research indicates that self-disclosure leads to close relationships; however, coping research indicates that dwelling on negative topics leads to emotional difficulties. Co-rumination is a single construct that integrates both perspectives and is proposed to be related both to positive friendship adjustment and problematic emotional adjustment. Third-, fifth-, seventh-, and ninth-grade participants (N = 608) responded to questionnaires, including a new measure of co-rumination. Co-rumination was related to high-quality, close friendships and aspects of depression and anxiety. Girls reported co-ruminating more than did boys, which helped to account for girls' more positive friendship adjustment and greater internalizing symptoms. Other analyses addressed whether co-rumination and the related constructs of self-disclosure and rumination had different relations with friendship and emotional adjustment.