Metabolic syndrome (MS), conventionally defined by the presence of at least three out of five dysmetabolic traits (abdominal obesity, hypertension, low plasma HDL-cholesterol, high plasma glucose and high triglycerides), has been associated with an increased risk of several age-related chronic diseases, including breast cancer (BC). This may have prognostic implications for BC survivors. 2,092 early stage BC survivors aged 35-70, recruited in eleven Italian centres 0-5 years after surgical treatment (1.74 years on average), were followed-up over 2.8 years on average for additional BC-related events, including BC-specific mortality, distant metastasis, local recurrences and contralateral BC. At recruitment, 20 % of the patients had MS. Logistic regression models were carried out to generate OR and 95 % confidence intervals (CI) for new BC events associated with MS, adjusting for baseline pathological prognostic factors. New BC events occurred in 164 patients, including 89 distant metastases. The adjusted ORs for women with MS versus women without any MS traits were 2.17 (CI 1.31-3.60) overall, and 2.45 (CI 1.24-4.82) for distant metastasis. The OR of new BC events for women with only one or two MS traits was 1.40 (CI 0.91-2.16). All MS traits were positively associated with new BC events, and significantly so for low HDL and high triglycerides. MS is an important prognostic factor in BC. As MS is reversible through lifestyle changes, interventions to decrease MS traits in BC patients should be implemented in BC clinics.