Background: Hormone receptor positive breast cancer is characterized by the potential for disease recurrence many years after initial diagnosis. Endocrine therapy has been shown to reduce the risk of such recurrence, but the optimal duration of endocrine therapy remains unclear. Methods: We conducted a systematic review and meta-analysis to quantify the relative and absolute benefits and harms of extended adjuvant tamoxifen (>5 years of therapy) compared with adjuvant tamoxifen (≤5 years of therapy). Odds ratios (ORs), 95% confidence intervals (CIs), absolute risks, and the number needed to treat (NNT) were computed for pre-specified events including disease recurrence, distant recurrence, all-cause death, endometrial carcinoma, cardiovascular death and treatment discontinuation. Subgroup analyses by timing of recurrence and baseline lymph node and menopause status were carried out. Results: Six trials comprising 25,326 patients were included. Extended adjuvant tamoxifen was associated with a non-significant reduction in the risk of recurrence (OR 0.89, 95% CI 0.77-1.04, p=0.14, NNT 74). Similar results were seen for distant recurrence (OR 0.88, 95% CI 0.75-1.04, p=0.14, NNT 70). There was no association between extended adjuvant tamoxifen and all-cause death (OR 1.06, 95% CI 0.86-1.31, p=0.58). There was no reduction in risk during extended adjuvant therapy (i.e. between years 5 and 9), but a potential reduction in the risk of recurrence after completion of extended adjuvant tamoxifen (i.e. beyond 10 years after diagnosis). Subgroup analysis suggested benefit in lymph node positive patients. Endometrial carcinoma was substantially more frequent with extended adjuvant tamoxifen (OR 1.81, 95% CI 1.45-2.25, p<0.001, NNT 102), but among those with endometrial carcinoma, the odds of death were lower among the extended tamoxifen group (OR 0.50, 95% CI 0.28-0.90, p=0.02). Conclusions: Extended adjuvant tamoxifen is not associated with a significant reduction in recurrence or death in unselected patients. Patients with lymph node positive breast cancer may derive more benefit. Reduction in the risk of recurrence only appears after completion of extended adjuvant therapy.