In this retrospective study we assessed the expression of the HER2/neu oncogene product in a series of 574 consecutive breast cancer cases, all recruited at the Maurizio Ascoli Cancer Center of Civico Hospital, in Palermo, between January 1998 and June 2003. The HER2/neu expression was evaluated using immunohistochemistry and scored from 0 to +3 as per FDA recommendations. The HER2/neu expression levels were related to the clinical-pathological features of the disease, including tumor size, nodal and menopausal status, estrogen and progesterone receptors, and hormonal or chemotherapeutic treatment. In 108 patients with a follow-up period of 3 years or more, the HER2/neu expression was also related to their survival characteristics. A significant correlation (P = 0.011) between HER2/neu +3 and estrogen receptor-negative cases was observed in the 487 M0 patients. In addition, HER2/neu +3 cases were associated with a positive nodal status (57.4%), although this association was not quite significant (P = 0.06). More importantly, follow-up data revealed that, in the 91 M0 patients with an average follow-up period of 37 months, the percentage of HER2/neu +3 patients who relapsed was remarkably greater (54.8%) than that observed for the HER2/neu +1/0 cases when combined (34.2%). Furthermore, the disease-free interval (DFI) was 47 months in the HER2/neu +1/0 group, while it dropped to 45 months in c-HER2/neu +3 cases. Although the limited number of cases does not allow us to draw any definitive conclusions, our data suggest that high expression levels of HER2/neu +3 are associated with an early relapse and a shorter disease-free interval in M0 breast cancer patients.