OBJECTIVE: Although there are some randomized controlled trials that highlight the positive role of family-focused treatment added to pharmacotherapy in bipolar disorder, no trials using contemporary methodologies have analyzed the specific effect of working with caregiver-only groups. The aim of this study was to assess the efficacy of a psychoeducational group intervention focused on caregivers of euthymic bipolar patients. METHOD: A total of 113 medicated euthymic bipolar outpatients who lived with their caregivers were randomized into an experimental and a control group. Caregivers in the experimental group received twelve 90-min group psychoeducation sessions focused on knowledge of bipolar disorder and training in coping skills. The patients did not attend the groups. Caregivers assigned to the control group did not receive any specific intervention. Patients were assessed monthly during both the intervention and the 12 months of follow-up. The primary outcome was time to any mood recurrence. RESULTS: Psychoeducation group intervention focused on the caregivers of bipolar patients carried a reduction of the percentage of patients with any mood recurrence (chi2 = 6.53; p = 0.011) and longer relapse-free intervals (log-rank chi(2) = 4.04; p = 0.044). When different types of episodes were analyzed separately, the effect was significant for both the number of patients who experienced a hypomanic/manic recurrence (chi2 = 5.65; p = 0.017) and the time to such an episode (log-rank chi2 = 5.84; p = 0.015). The differences in preventing depressive and mixed episodes were not significant. CONCLUSIONS: A psychoeducation group intervention for the caregivers of bipolar patients is a useful adjunct to usual treatment for the patients in reducing the risk of recurrences, particularly mania and hypomania, in bipolar disorder.