CONTEXT:Mindfulness-based cognitive therapy (MBCT) is a group-based psychosocial intervention designed to enhance self-management of prodromal symptoms associated with depressive relapse. OBJECTIVE: To compare rates of relapse in depressed patients in remission receiving MBCT against maintenance antidepressant pharmacotherapy, the current standard of care. DESIGN: Patients who met remission criteria after 8 months of algorithm-informed antidepressant treatment were randomized to receive maintenance antidepressant medication, MBCT, or placebo and were followed up for 18 months. SETTING: Outpatient clinics at the Centre for Addiction and Mental Health, Toronto, Ontario, Canada, and St Joseph's Healthcare, Hamilton, Ontario. PARTICIPANTS: One hundred sixty patients aged 18 to 65 years meeting DSM-IV criteria for major depressive disorder with a minimum of 2 past episodes. Of these, 84 achieved remission (52.5%) and were assigned to 1 of the 3 study conditions. INTERVENTIONS: Patients in remission discontinued their antidepressants and attended 8 weekly group sessions of MBCT, continued taking their therapeutic dose of antidepressant medication, or discontinued active medication and were switched to placebo. MAIN OUTCOME MEASURE: Relapse was defined as a return, for at least 2 weeks, of symptoms sufficient to meet the criteria for major depression on module A of the Structured Clinical Interview for DSM-IV. RESULTS: Intention-to-treat analyses showed a significant interaction between the quality of acute-phase remission and subsequent prevention of relapse in randomized patients (P = .03). Among unstable remitters (1 or more Hamilton Rating Scale for Depression score >7 during remission), patients in both MBCT and maintenance treatment showed a 73% decrease in hazard compared with placebo (P = .03), whereas for stable remitters (all Hamilton Rating Scale for Depression scores ≤7 during remission) there were no group differences in survival. CONCLUSIONS: For depressed patients achieving stable or unstable clinical remission, MBCT offers protection against relapse/recurrence on a par with that of maintenance antidepressant pharmacotherapy. Our data also highlight the importance of maintaining at least 1 long-term active treatment in unstable remitters.