Receiving a diagnosis and undergoing cancer treatment is associated with high levels of emotional distress and related symptoms (e.g., anxiety, depression, and fatigue). MBSR has been consistently shown to reduce symptoms of stress and mood disturbance among cancer patients. Despite this, the question of “how” MBSR works has not been adequately addressed; it is not yet known whether targeted constructs are critical in changing outcomes. Given evidence of associations among mindfulness, emotion regulation difficulties, and psychological outcomes, it is plausible that MBSR may affect outcomes through increased mindfulness and improved emotion regulation—this hypothesis remains to be tested. The current study is a longitudinal waitlist controlled trial that aims to identify whether theoretically proposed mediators account for the established psychological benefits of participating in an MBSR program, among cancer patients. Six consecutive 8-week MBSR programs spaced several months apart will be used. Patients will be recruited from those already on the waitlist for the MBSR program. Participants will be asked to complete a questionnaire package three times: prior to, half-way through, and following the MBSR program if they are in the immediate group, or at baseline, 4 weeks, and 8 weeks if they are in the waiting group. Hierarchical linear modeling (HLM) will be employed to assess potential changes over time as a function of condition (intervention vs. control) for the pre-, mid-, and post-intervention data obtained for psychological outcome variables. Following recommended procedures for assessing mediation, a series of linear regression analyses will be conducted to assess whether proposed mediators fully or partially mediate change in symptoms of stress and mood disturbance. Data will be collected between January 2008 and June 2009. We are currently in the process of recruiting patients for this study, and anticipate being in a position to report the results by August 2009.