Same-sex friendships are an important source of social support and typically contribute to positive adjustment. However, there can be adjustment trade-offs if the friends co-ruminate (i.e., talk excessively about problems) in that co-rumination is related to having close friendships but also to increased internalizing symptoms. The current study utilized an experimental manipulation that elicited co-rumination in young women and thus mirrored an everyday response to stress. Observed co-rumination was associated with a significant increase in the stress hormone, cortisol (after controlling for self-reported co-rumination and for cortisol levels assessed before the discussion of problems). These findings suggest that co-rumination can amplify, rather than mitigate, the hormonal stress response to personal life stressors.