The current study examined co-rumination (i.e., extensively discussing, rehashing, and speculating about problems) in the context of mother-adolescent relationships. Fifth-, eighth-, and eleventh-graders (N=516) reported on co-rumination and more normative self-disclosure with mothers, their relationships with mothers, and their own internalizing symptoms. A subset of mothers (N=200) reported on mother-adolescent co-rumination and self-disclosure. Results from the adolescent-report data indicated greater mother-adolescent co-rumination with daughters than sons and also adjustment trade-offs of mother-adolescent co-rumination. Mother-adolescent co-rumination was related to positive relationship quality but also to enmeshment in the relationship. Whereas the relation with positive relationship quality appeared to be due in part to normative self-disclosure, the relation with enmeshment was unique to co-rumination. Mother-adolescent co-rumination also was related to youth anxiety/depression. The relations with enmeshment and internalizing symptoms were strongest when co-rumination focused on the mothers' problems. Implications of mother-adolescent co-rumination for promoting appropriate relationship boundaries and youth well-being are discussed.