To assess the prognostic value of presurgical CA15.3 in a large cohort of patients with early breast cancer. A total of 7.942 consecutive patients with breast cancer operated at the European Institute of Oncology between 1998 and 2005 and with presurgical values of CA 15.3 available were included. We explored patterns of recurrence by baseline CA 15.3 values. Mean CA15.3 was 17.0 U/ml. CA15.3 was associated with age, tumor size, nodal involvement, Ki-67 labeling index, grade, HER2 expression, molecular subtype, and perivascular invasion. CA15.3 was independently associated with distant metastases [HR > 20 U/ml vs. ≤ 20 U/ml: 1.34 (95% CI 1.15-1.56)] and death [HR > 20 U/ml vs. ≤ 20 U/ml: 1.30 (95% CI 1.11-1.53)]. When considering CA15.3 as continuous variable, we observed a constant risk of metastasis and death from the lowest values to about 15-20 U/ml, and then a significantly increasing risk with increasing values of CA15.3. Finally, CA15.3 provided significant additional information to the common prognostic factors to predict the occurrence of metastases (C-index P value 0.04). In patients with operable breast cancer, presurgical CA15.3 value is an independent prognostic factor for metastases and deaths. CA15.3 provides additional information to the common prognostic factors and should be considered in the adjuvant therapeutic algorithm.