In order to evaluate the potential risk factors for lymph node metastasis in invasive breast cancer patients submitted to axillary dissection, 675 patients who received surgery between January 1995 and December 2002 were included in a prospective study. In all cases, MIB-1 proliferation index was investigated by immunohistochemistry. Lymph node involvement was found in 248 out of 675 patients. Univariate analysis showed that peritumoral lymphovascular invasion, pT stage, tumor multiplicity, MIB-1 proliferation index >10%, oestrogen receptor status, histological type, tumor grade and progesterone receptor status were related to a higher incidence of lymph node metastasis, with various levels of statistical significance. Multivariate analysis identified lymphovascular invasion [relative risk (RR, 7.69; p<0.001), pT stage (RR, 3.08; p<0.001), tumor multiplicity (RR, 3.89; p<0.001), and MIB-1 proliferation index (RR, 1.66; p=0.019)] as independent predictive variables. The impact of MIB-1 positivity on the incidence of lymph node metastasis was particularly evident in intermediate risk groups (pT1c, pT2 without lymphovascular invasion), as well as in grade-2 tumors. In conclusion, the MIB-1 proliferation index could provide additional information about the risk of lymph node metastasis in invasive breast cancer, and may be useful to identify grade-2 tumors with a more aggressive clinical behaviour.