['Mindfulness-based stress reduction (MBSR) has demonstrated efficacy for alleviating cancer-related distress. Although theorized to be the means by which people improve, it is yet to be determined whether outcomes are related to the development or enhancement of mindfulness among participants. This study examined the effect of participation in an MBSR program on levels of mindfulness in a heterogeneous sample of individuals with cancer, and if these changes were related to improvements in stress and mood outcomes.', 'In all, 268 individuals with cancer completed self-report assessments of stress and mood disturbances before and after participation in an 8-week MBSR program. Of these, 177 participants completed the Mindful Attention Awareness Scale and 91 participants completed the Five Facet Mindfulness Questionnaire, at both time points.', 'Levels of mindfulness on both measures increased significantly over the course of the program. These were accompanied by significant reductions in mood disturbance (55%) and symptoms of stress (29%). Increases in mindfulness accounted for a significant percentage of the reductions in mood disturbance (21%) and symptoms of stress (14%). Being aware of the present moment and refraining from judging inner experience were the 2 most important mindfulness skills for improvements of psychological functioning among cancer patients.'] Is increased mindfulness related to improved stress and mood following participation in a mindfulness-based stress reduction program in individuals with cancer?