Mounting research shows that the tendency to co-ruminate with peers regarding ongoing problems increases adolescents' depression risk; however, the means by which this interpersonal process fosters risk has not been identified. This said, theorists have proposed that co-rumination increases depression risk, in part, by increasing one's tendency to ruminate when alone. We tested this hypothesis in a study of 201 high-school freshmen who completed two assessments, six months apart. Supporting the proposed model, co-rumination predicted prospective increases in rumination and rumination predicted increases in depressive symptoms. The direct effect of co-rumination on depressive symptom change was not significant. Results indicate that co-rumination with friends may serve to increase rumination, which in turn increases depression risk.