Introduction: As the incidence of and survival from breast cancer continue to raise, interventions to reduce anxiety and depression before, during and after treatment are needed. Previous studies have reported positive effects of a structured 8-week group mindfulness-based stress reduction program (MBSR) among patients with cancer and other conditions. Purpose: To test the effect of such a programme on anxiety and depression among women with breast cancer in a population-based randomised controlled study. Methods: A total of 336 women who had been operated on for breast cancer (stage I-III) were randomised to usual care or MBSR + usual care. Questionnaires including the Symptom Checklist-90r anxiety and depression subscales and the Center for Epidemiological Studies-Depression scale were administered before randomisation and immediately, 6 and 12 months after the intervention. Results: Intention-to-treat analyses showed differences between groups in levels of anxiety (p = 0.0002) and depression (SCL-90r, p < 0.0001; CES-D, p = 0.0367) after 12 months. Graphical comparisons of participants with higher levels of anxiety and depression at baseline showed a significantly greater decrease in the intervention group throughout follow-up and no differences among least affected participants. Medium-to-large effects were found for all outcomes in the intervention group in analyses of change scores after 12 months' follow-up. Conclusion: The 8-week group based MBSR intervention had clinically meaningful, statistically significant effects on depression and anxiety after 12 months' follow-up, and medium-to-large effect sizes. Our findings support the dissemination of MBSR among women with breast cancer. (Clintrials.gov No.: NCT00990977).