Mindfulness-based stress reduction (MBSR) has shown effectiveness for a variety of mental health conditions. However, it is not known for whom the intervention is most effective. In a randomized controlled trial (N = 30), we explored whether individuals with higher levels of pretreatment trait mindfulness would benefit more from MBSR intervention. Results demonstrated that relative to a control condition (n = 15), MBSR treatment (n = 15) had significant effects on several outcomes, including increased trait mindfulness, subjective well-being, and empathy measured at 2 and 12 months after treatment. However, relative to controls, MBSR participants with higher levels of pretreatment mindfulness showed a larger increase in mindfulness, subjective well-being, empathy, and hope, and larger declines in perceived stress up to 1 year after treatment. © 2010 Wiley Periodicals, Inc. J Clin Psychol 00:1–11, 2011.