BACKGROUND:There is increasing recognition of mindfulness and mindfulness training as a way to decrease stress and increase psychological functioning. PURPOSE: The aims of this study were to examine the effects of mindfulness stress reduction training on perceived stress and psychological well-being and to examine if changes in mindfulness mediate intervention effects on these outcomes. METHODS: Seventy women and one man with a previous cancer diagnosis (mean age 51.8 years, standard deviation = 9.86) were randomized into an intervention group or a wait-list control group. The intervention consisted of an 8-week mindfulness training course. RESULTS: Compared to participants in the control group, participants in the mindfulness training group had significantly decreased perceived stress and posttraumatic avoidance symptoms and increased positive states of mind. Those who participated in the intervention reported a significant increase in scores on the five-facet mindfulness questionnaire (FFMQ) when compared to controls. The increase in FFMQ score mediated the effects of the intervention on perceived stress, posttraumatic avoidance symptoms, and positive states of mind. CONCLUSIONS: This study indicates that the improvements in psychological well-being resulting from mindfulness stress reduction training can potentially be explained by increased levels of mindfulness as measured with the FFMQ. The importance of these findings for future research in the field of mindfulness is discussed.