The proto-oncogene pituitary tumor-transforming gene (PTTG) is described as abundantly overexpressed in a variety of neoplasms and cancer cell lines. We analyzed PTTG mRNA expression in 72 tumor samples derived from primary tumors of patients suffering from breast cancer and in unaffected breast epithelium, checking for correlations between expression levels and standard clinico-pathological parameters over a 5-year observation period. We found a direct correlation between PTTG mRNA overexpression and lymph node infiltration. Furthermore, overexpression of this gene in tumors correlated with a higher degree of tumor recurrence within the subsequent observation period. Taken together, these results demonstrate that determination of PTTG expression in primary tumors of the breast is a powerful tool for the assessment of potential tumor aggressiveness.