OBJECTIVE: Mindfulness meditation has met increasing interest as a therapeutic strategy for anxiety disorders, but prior studies have been limited by methodological concerns, including a lack of an active comparison group. This is the first randomized, controlled trial comparing the manualized Mindfulness-Based Stress Reduction (MBSR) program with an active control for generalized anxiety disorder (GAD), a disorder characterized by chronic worry and physiologic hyperarousal symptoms. METHOD: Ninety-three individuals with DSM-IV-diagnosed GAD were randomly assigned to an 8-week group intervention with MBSR or to an attention control, Stress Management Education (SME), between 2009 and 2011. Anxiety symptoms were measured with the Hamilton Anxiety Rating Scale (HAMA; primary outcome measure), the Clinical Global Impressions-Severity of Illness and -Improvement scales (CGI-S and CGI-I), and the Beck Anxiety Inventory (BAI). Stress reactivity was assessed by comparing anxiety and distress during pretreatment and posttreatment administration of the Trier Social Stress Test (TSST). RESULTS: A modified intent-to-treat analysis including participants who completed at least 1 session of MBSR (n = 48) or SME (n = 41) showed that both interventions led to significant (P < .0001) reductions in HAMA scores at endpoint, but did not significantly differ. MBSR, however, was associated with a significantly greater reduction in anxiety as measured by the CGI-S, the CGI-I, and the BAI (all P values < .05). MBSR was also associated with greater reductions than SME in anxiety and distress ratings in response to the TSST stress challenge (P < .05) and a greater increase in positive self-statements (P = .004). CONCLUSIONS: These results suggest that MBSR may have a beneficial effect on anxiety symptoms in GAD and may also improve stress reactivity and coping as measured in a laboratory stress challenge. TRIAL REGISTRATION: ClinicalTrials.gov identifier: NCT01033851.