Abstract Background. Based on the hypothesis that impaired glucose metabolism might be associated with survival outcomes independently of overt diabetes, we sought to investigate the prognostic value of routinely used glycemic parameters in a prospective study of breast cancer (BC) patients. Patients and Methods. Fasting blood glucose, insulin and HbA1c levels, and insulin resistance (assessed by the Homeostasis Model Assessment [HOMA] index) at diagnosis were evaluated in 286 nondiabetic BC patients (249 with primary cancer, 37 with metastatic) with respect to those parameters’ possible associations with clinicopathological features and survival outcomes. As a control group, 143 healthy women matched in a 2:1 ratio for age, blood lipid levels, and body mass index were also investigated. Results. Fasting blood glucose level (mean ± SD: 99 ± 26 vs. 85 ± 15 mg/dL), insulin level (median: 10.0 vs. 6.8 μIU/mL), and HOMA index (median: 2.2 vs. 1.4), but not HbA1c level, were significantly elevated in BC patients compared with control subjects. Receiver operating characteristics analysis showed comparable areas for blood glucose and insulin levels, and HOMA index (ranging from 0.668 to 0.671). Using a cutoff level of 13 μIU/mL, insulin had the best specificity (92%) and sensitivity (41%), was significantly associated with disease stage, and acted as a negative prognostic marker of progression-free survival (hazard ratio: 2.17; 95% confidence interval: 1.13–4.20) independently of menopausal status, disease stage, hormone receptor status, and human epidermal growth factor receptor 2 and Ki67 expression. Conclusion. These results suggest that insulin determination might provide prognostic information in BC and support the hypothesis that lifestyle and/or pharmacological interventions targeting glucose metabolism could be considered to improve survival outcome of selected BC patients.