OBJECTIVES:The current study assessed associations between changes in 5 facets of mindfulness (Acting With Awareness, Observing, Describing, Non-Reactivity, and Nonjudgment) and changes in 4 posttraumatic stress disorder (PTSD) symptom clusters (Re-Experiencing, Avoidance, Emotional Numbing, and Hyperarousal symptoms) among veterans participating in mindfulness-based stress reduction (MBSR). METHOD: Secondary analyses were performed with a combined data set consisting of 2 published and 2 unpublished trials of MBSR conducted at a large Veterans Affairs hospital. The combined sample included 113 veterans enrolled in MBSR who screened positive for PTSD and completed measures of mindfulness and PTSD symptoms before and after the 8-week intervention. RESULTS: Increases in mindfulness were significantly associated with reduced PTSD symptoms. Increases in Acting With Awareness and Non-Reactivity were the facets of mindfulness most strongly and consistently associated with reduced PTSD symptoms. Increases in mindfulness were most strongly related to decreases in Hyperarousal and Emotional Numbing. CONCLUSIONS: These results extend previous research, provide preliminary support for changes in mindfulness as a viable mechanism of treatment, and have a number of potential practical and theoretical implications.