The purpose of this study was to extend the research on co-rumination and depressed mood by examining the impact of co-rumination on depressed mood on a daily basis while controlling for the effects of daily stress events in a sample of late adolescents. Two-hundred and seventy-nine predominantly Caucasian college students (95 male, 184 female) completed a baseline assessment as well as morning and evening online surveys for 7 days as part of a daily diary study. Baseline assessment measured co-rumination and depressive symptoms, whereas morning surveys measured depressed mood and evening surveys included questions about depressed mood, within-day co-rumination, and daily stressful life events. Data were analyzed using Multilevel Modeling due to the daily nature of the data. Women co-ruminated more than men on a daily basis. Daily co-rumination predicted within-day increases in depressed mood while controlling for stress. Baseline co-rumination moderated the relationship between daily stressful life events and depressed mood, but daily co-rumination did not moderate this relationship. Co-rumination with closest confidant is associated with within-day worsening of depressed mood. Future research should consider the impact of co-rumination with closest confidants in addition to same-sex best friend. Implications for treatment include active consideration and discussion of patient's coping and support-seeking behaviors and paying greater attention to the types of dialogue that occur within one's social support network. (Contains 3 tables and 1 figure.)