BACKGROUND: An appreciation of cognitive predictors of change in treatment outcome may help to better understand differential treatment outcomes. The aim of this study was to examine how rumination and mindfulness impact on treatment outcome in two group-based interventions for non-melancholic depression: Cognitive Behaviour Therapy (CBT) and Mindfulness-Based Cognitive Therapy (MBCT). METHOD: Sixty-nine participants were randomly allocated to either 8-weekly sessions of group CBT or MBCT. Complete data were obtained from 45 participants (CBT = 26, MBCT = 19). Outcome was assessed at completion of group treatments. RESULTS: Depression scores improved for participants in both group interventions, with no significant differences between the two treatment conditions. There were no significant differences between the interventions at post-treatment on mindfulness or rumination scores. Rumination scores significantly decreased from pre- to post-treatment for both conditions. In the MBCT condition, post-treatment rumination scores were significantly associated with post-treatment mindfulness scores. CONCLUSIONS: Results suggest that decreases in rumination scores may be a common feature following both CBT and MBCT interventions. However, post-treatment rumination scores were associated with post-treatment mindfulness in the MBCT condition, suggesting a unique role for mindfulness in understanding treatment outcome for MBCT.