Posttraumatic stress disorder (PTSD) affects 23% of U.S. veterans returning from deployment in Afghanistan and Iraq.1 Left untreated PTSD is associated with high rates of comorbidity, disability, and poor quality of life.2 Although existing first line treatments such as prolonged exposure and cognitive processing therapy are efficacious, 30–50% of veterans inadequately respond to these techniques and completion rates are low.3, 4 Mindfulness-based stress reduction (MBSR) is a widely used therapeutic strategy that trains patients to attend to the present moment and embrace it non-judgmentally. Some evidence supports its effectiveness for symptoms of anxiety and depression,5 and learning to accept painful thoughts and feelings, rather than completely avoiding them, targets a key contributing factor in the development and maintenance of PTSD.6In the present trial, 116 veterans (mean age 58.5 years) with PTSD were randomized to receive weekly sessions of either grouped-based MBSR or present-centered group therapy (PCT), which served as an active control.7 MBSR consisted of eight sessions (2.5 h each) plus one 6.5-h silent retreat. Sessions included didactic training and practice in three meditation techniques: body scan, seated contemplation, and mindfulness yoga. PCT, which consisted of nine sessions (1.5 h each), is a credible intervention shown to be beneficial for PTSD.8, 9 Veterans currently suffering from substance dependence, psychotic disorder, prominent suicidal or homicidal ideation, or cognitive impairment that would interfere with treatment were excluded. In an intention-to-treat analysis, the mean difference in self-reported symptom severity score was 4.95 (95% CI: 1.92–7.99) and 6.44 (3.34–9.53) at nine weeks and two months follow-up, respectively, favoring MBSR. The minimal clinically important difference for this scale is reportedly 10 (scale range: 17–85 with higher scores indicating more severe symptoms).10 The proportion of MBSR patients exhibiting a clinically important improvement in symptoms was 36.5% and 48.9% at nine weeks and two months, respectively. Between-group differences (vs. PCT) were 13.7% (95% CI: −3.5 to 31.0) and 20.9% [2.2–39.5; number needed to treat (NNT) = 5]. Changes in interview-rated PTSD severity or depressive symptoms were not significant at either time point. MBSR showed a modest reduction in self-reported PTSD symptoms compared to present-centered therapy, an active control that served to reduce the risk of performance bias. Although dropout rates were substantially higher in the MBSR group (22.4% vs. 6.9%), they were lower than those typically found in trials investigating prolonged exposure and cognitive processing therapy.3, 4, 9 Limitations of this trial include short duration and differences in baseline PTSD severity (MBSR > PCT). The fact that 75% of the participants were Vietnam-era veterans and 97% were white may restrict the generalizability of these results.