OBJECTIVE: To compare the efficacy of mindfulness-based cognitive therapy (MBCT) plus treatment as usual (TAU) to TAU alone for patients with bipolar disorder over a 12-month follow-up period. METHOD: Participants with a DSM-IV diagnosis of bipolar disorder were randomly allocated to either MBCT plus TAU or TAU alone. Primary outcome measures were time to recurrence of a DSM-IV major depressive, hypomanic or manic episode; the Montgomery-Åsberg Depression Rating Scale (MADRS); and Young Mania Rating Scale (YMRS). Secondary outcome measures were number of recurrences, the Depression Anxiety Stress Scales (DASS), and the State Trait Anxiety Inventory (STAI). RESULTS: Ninety-five participants with bipolar disorder were recruited to the study (MBCT = 48; TAU = 47). Intention-to-treat (ITT) analysis found no significant differences between the groups on either time to first recurrence of a mood episode or total number of recurrences over the 12-month period. Furthermore, there were no significant between-group differences on the MADRS or YMRS scales. A significant between-group difference was found in STAI - state anxiety scores. There was a significant treatment by time interaction for the DAS - achievement subscale. CONCLUSION: While MBCT did not lead to significant reductions in time to depressive or hypo/manic relapse, total number of episodes, or mood symptom severity at 12-month follow-up, there was some evidence for an effect on anxiety symptoms. This finding suggests a potential role of MBCT in reducing anxiety comorbid with bipolar disorder.