High circulating glucose has been associated with increased risk of breast cancer (BC). There may also be a link between serum glucose and prognosis in women treated for BC. We assessed the effect of peridiagnostic fasting blood glucose and body mass index (BMI) on long-term BC prognosis. We retrospectively investigated 1,261 women diagnosed and treated for stage I-III BC at the National Cancer Institute, Milan, in 1996, 1999 and 2000. Data on blood tests and follow-up were obtained by linking electronic archives, with follow-up to end of 2009. Multivariate Cox modelling estimated hazard ratios (HR) with 95 % confidence intervals (CI) for distant metastasis, recurrence and death (all causes) in relation to categorized peridiagnostic fasting blood glucose and BMI. Mediation analysis investigated whether blood glucose mediated the BMI-breast cancer prognosis association. The risks of distant metastasis were significantly higher for all other quintiles compared to the lowest glucose quintile (reference <87 mg/dL) (respective HRs: 1.99 95 % CI 1.23-3.24, 1.85 95 % CI 1.14-3.0, 1.73 95 % CI 1.07-2.8, and 1.91 95 % CI 1.15-3.17). The risk of recurrence was significantly higher for all other glucose quintiles compared to the first. The risk of death was significantly higher than reference in the second, fourth and fifth quintiles. Women with BMI ≥ 25 kg/m(2) had significantly greater risks of recurrence and distant metastasis than those with BMI < 25 kg/m(2), irrespective of blood glucose. The increased risks remained invariant over a median follow-up of 9.5 years. Mediation analysis indicated that glucose and BMI had independent effects on BC prognosis. Peridiagnostic high fasting glucose and obesity predict worsened short- and long-term outcomes in BC patients. Maintaining healthy blood glucose levels and normal weight may improve prognosis.