BACKGROUND: Breast cancer is the most common cancer in women. Diagnosis and treatment may drastically affect quality of life, causing symptoms such as sleep disorders, depression and anxiety. Mindfulness-based stress reduction (MBSR) is a programme that aims to reduce stress by developing mindfulness, meaning a non-judgmental, accepting moment-by-moment awareness. MBSR seems to benefit patients with mood disorders and chronic pain, and it may also benefit women with breast cancer. OBJECTIVES: To assess the effects of mindfulness-based stress reduction (MBSR) in women diagnosed with breast cancer. SEARCH METHODS: In April 2018, we conducted a comprehensive electronic search for studies of MBSR in women with breast cancer, in the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, Embase, and two trial registries (World Health Organization's International Clinical Trials Registry Platform (WHO ICTRP) and ClinicalTrials.gov). We also handsearched relevant conference proceedings. SELECTION CRITERIA: Randomised clinical trials (RCTs) comparing MBSR versus no intervention in women with breast cancer. DATA COLLECTION AND ANALYSIS: We used standard methodological procedures expected by Cochrane. Using a standardised data form, the review authors extracted data in duplicate on methodological quality, participants, interventions and outcomes of interest (quality of life, fatigue, depression, anxiety, quality of sleep, overall survival and adverse events). For outcomes assessed with the same instrument, we used the mean difference (MD) as a summary statistic for meta-analysis; for those assessed with different instruments, we used the standardised mean difference (SMD). The effect of MBSR was assessed in the short term (end of intervention), medium term (up to 6 months after intervention) and long term (up to 24 months after intervention). MAIN RESULTS: Fourteen RCTs fulfilled our inclusion criteria, with most studies reporting that they included women with early breast cancer. Ten RCTs involving 1571 participants were eligible for meta-analysis, while four studies involving 185 participants did not report usable results. Queries to the authors of these four studies were unsuccessful. All studies were at high risk of performance and detection bias since participants could not be blinded, and only 3 of 14 studies were at low risk of selection bias. Eight of 10 studies included in the meta-analysis recruited participants with early breast cancer (the remaining 2 trials did not restrict inclusion to a certain cancer type). Most trials considered only women who had completed cancer treatment.MBSR may improve quality of life slightly at the end of the intervention (based on low-certainty evidence from three studies with a total of 339 participants) but may result in little to no difference up to 6 months (based on low-certainty evidence from three studies involving 428 participants). Long-term data on quality of life (up to two years after completing MBSR) were available for one study in 97 participants (MD 0.00 on questionnaire FACT-B, 95% CI -5.82 to 5.82; low-certainty evidence).In the short term, MBSR probably reduces fatigue (SMD -0.50, 95% CI -0.86 to -0.14; moderate-certainty evidence; 5 studies; 693 participants). It also probably slightly reduces anxiety (SMD -0.29, 95% CI -0.50 to -0.08; moderate-certainty evidence; 6 studies; 749 participants), and it reduces depression (SMD -0.54, 95% CI -0.86 to -0.22; high-certainty evidence; 6 studies; 745 participants). It probably slightly improves quality of sleep (SMD -0.38, 95% CI -0.79 to 0.04; moderate-certainty evidence; 4 studies; 475 participants). However, these confidence intervals (except for short-term depression) are compatible with both an improvement and little to no difference.In the medium term, MBSR probably results in little to no difference in medium-term fatigue (SMD -0.31, 95% CI -0.84 to 0.23; moderate-certainty evidence; 4 studies; 607 participants). The intervention probably slightly reduces anxiety (SMD -0.28, 95% CI -0.49 to -0.07; moderate-certainty evidence; 7 studies; 1094 participants), depression (SMD -0.32, 95% CI -0.58 to -0.06; moderate-certainty evidence; 7 studies; 1097 participants) and slightly improves quality of sleep (SMD -0.27, 95% CI -0.63 to 0.08; moderate-certainty evidence; 4 studies; 654 participants). However, these confidence intervals are compatible with both an improvement and little to no difference.In the long term, moderate-certainty evidence shows that MBSR probably results in little to no difference in anxiety (SMD -0.09, 95% CI -0.35 to 0.16; 2 studies; 360 participants) or depression (SMD -0.17, 95% CI -0.40 to 0.05; 2 studies; 352 participants). No long-term data were available for fatigue or quality of sleep.No study reported data on survival or adverse events. AUTHORS' CONCLUSIONS: MBSR may improve quality of life slightly at the end of the intervention but may result in little to no difference later on. MBSR probably slightly reduces anxiety, depression and slightly improves quality of sleep at both the end of the intervention and up to six months later. A beneficial effect on fatigue was apparent at the end of the intervention but not up to six months later. Up to two years after the intervention, MBSR probably results in little to no difference in anxiety and depression; there were no data available for fatigue or quality of sleep.