An immunohistochemical determination of the growth fraction (GF) with the Ki-67 monoclonal antibody has been performed in a prospective series of 140 patients with primary operable breast carcinoma. GF ranged from 0% to 43% Ki-67 stained cells with a median value of 8%. High GF (> 8%) was significantly associated with axillary node involvement (p = 0.006), aneuploidy (p = 0.008), histologic grade (p = 0.03), and S-phase fraction > 5% determined by flow cytometry (p = 0.01). After a median follow-up of 6 years, the univariate analysis did not show significant correlation between high GF and worse relapse-free survival (p = 0.10) or shorter overall survival. However, a multivariate analysis on relapse-free survival, performed in 127 comparable patients, showed that GF was an independent predictive factor (p = 0.03) together with nodal status (p = 0.00001), age under 45 years (p = 0.0008), and chemotherapy (0.006). In node negative patients, GF was still an independent prognostic indicator (p = 0.002) together with age under 45 years (p = 0.0003). Tumor proliferative activity evaluated by the monoclonal antibody Ki-67 appears to be an effective indicator of prognosis in breast cancer and could be of assistance in the decision making of adjuvant therapy in node negative patients.