BACKGROUNDAlthough mindfulness-based therapy has become a popular treatment, little is known about its efficacy. OBJECTIVES To conduct an effect size analysis of this popular intervention for anxiety and mood symptoms in clinical samples. DATA SOURCES A literature search was conducted using PubMed, PsycInfo, the Cochrane Library, and manual searches. REVIEW METHODS The search identified 39 studies totaling 1,140 participants receiving mindfulness-based therapy for a range of conditions, including cancer, generalized anxiety disorder, depression, and other psychiatric or medical conditions. RESULTS Effect size estimates suggest that mindfulness-based therapy was moderately effective for improving anxiety (Hedges’ g = 0.63) and mood symptoms (Hedges’ g = 0.59) from pre to post-treatment in the overall sample. In patients with anxiety and mood disorders, this intervention was associated with effect sizes (Hedges’ g) of 0.97 and 0.95 for improving anxiety and mood symptoms, respectively. These effect sizes were robust, unrelated to publication year or number of treatment sessions, and were maintained over follow-up. CONCLUSION These results suggest that mindfulness-based therapy is a promising intervention for treating anxiety and mood problems in clinical populations.