OBJECTIVE: In a pilot randomized controlled trial, examine the feasibility and preliminary efficacy of an 8-week, mindfulness training program (Mindfulness Based Stress Reduction) in a sample of men on active surveillance on important psychological outcomes including prostate cancer anxiety, uncertainty intolerance and posttraumatic growth. METHODS: Men were randomized to either mindfulness (n = 24) or an attention control arm (n = 19) and completed self-reported measures of prostate cancer anxiety, uncertainty intolerance, global quality of life, mindfulness and posttraumatic growth at baseline, 8 weeks, 6 months and 12 months. RESULTS: Participants in the mindfulness arm demonstrated significant decreases in prostate cancer anxiety and uncertainty intolerance, and significant increases in mindfulness, global mental health and posttraumatic growth. Participants in the control condition also demonstrated significant increases in mindfulness over time. Longitudinal increases in posttraumatic growth were significantly larger in the mindfulness arm than they were in the control arm. CONCLUSIONS: While mindfulness training was found to be generally feasible and acceptable among participants who enrolled in the 8-week intervention as determined by completion rates and open-ended survey responses, the response rate between initial enrollment and the total number of men approached was lower than desired (47%). While larger sample sizes are necessary to examine the efficacy of mindfulness training on important psychological outcomes, in this pilot study posttraumatic growth was shown to significantly increase over time for men in the treatment group. Mindfulness training has the potential to help men cope more effectively with some of the stressors and uncertainties associated with active surveillance. Copyright © 2016 John Wiley & Sons, Ltd.