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.