A ruminative response style has been shown to predict depressive symptoms among youth and adults, but it is unclear whether rumination is associated specifically with depression compared with co-occurring symptoms of anxiety and externalizing behaviors. This prospective, multiwave study investigated whether baseline rumination predicted prospective elevations in depressive symptoms specifically. Rumination was assessed at baseline in a sample of early and middle adolescents (N = 350, 6-10th graders). Symptom measures of depression, anxious arousal, general internalizing, and conduct/externalizing problems with good discriminant validity were assessed at four time points over a 5-month period. Results using hierarchical linear modeling show that rumination predicted prospective fluctuations in symptoms of depression and general internalizing problems specifically but not anxious arousal or externalizing problems. Rumination predicted increasing prospective trajectories of general internalizing symptoms. Baseline rumination interacted with prospective co-occurring fluctuations of anxious arousal and externalizing behaviors over time to predict the highest levels of prospective depressive symptoms. Rumination partly mediated the sex difference (girls greater than boys) in depressive and internalizing symptoms. (Contains 2 tables and 2 figures.)