This controlled pilot study tested the effects of a mindfulness intervention on obsessive compulsive disorder (OCD) symptoms and tested the psychological processes possibly mediating such effects. Participants with OCD symptoms (12 women, 5 men) received either mindfulness training (N=8) or formed a waiting-list control group (N = 9). Meditation included 8 group meetings teaching meditative breathing, body-scan, and mindful daily living, applied to OCD. The intervention had a significant and large effect on mindfulness, OCD symptoms, letting go, and thought-action fusion. Controlling for changes in "letting go," group effects on change in OCD symptoms disappeared, pointing at a mediating role for letting go. This may be the first controlled study demonstrating that a mindfulness intervention reduces OCD symptoms, possibly explained by increasing letting go capacity. If replicated in larger and clinical samples, mindfulness training may be an alternative therapy for OCD.