Co-ruminating, or excessively discussing problems, with friends is proposed to have adjustment tradeoffs. Co-rumination is hypothesized to contribute both to positive friendship adjustment and to problematic emotional adjustment. Previous single-assessment research was consistent with this hypothesis, but whether co-rumination is an antecedent of adjustment changes was unknown. A 6-month longitudinal study with middle childhood to midadolescent youths examined whether co-rumination is simultaneously a risk factor (for depression and anxiety) and a protective factor (for friendship problems). For girls, a reciprocal relationship was found in which co-rumination predicted increased depressive and anxiety symptoms and increased positive friendship quality over time, which, in turn, contributed to greater co-rumination. For boys, having depressive and anxiety symptoms and high-quality friendships also predicted increased co-rumination. However, for boys, co-rumination predicted only increasing positive friendship quality and not increasing depression and anxiety. An implication of this research is that some girls at risk for developing internalizing problems may go undetected because they have seemingly supportive friendships.