Cell-proliferation markers are very important in the clinical management of cancer patients, and the identification of Ki-67 (a monoclonal antibody that recognizes proliferating cells) can make it easier to define the level of proliferative activity. This study investigated the associations between the Ki-67 levels measured by means of immunohistochemistry, and other clinical and pathological variables and prognosis in 322 breast-cancer patients. A significant association was found (p < 0.001) between Ki-67 values and tumor size, nodal status, estrogen and progesterone receptor status; multivariate analysis showed that Ki-67 levels were associated with disease-free and overall survival, thus confirming that it is an independent prognostic variable. Various statistical approaches were used in an attempt to establish the best cut-off point for dividing patients into groups at high or low risk of relapse but, in this series, we could find no evidence leading to a single "best" cut-off point. We conclude that the quantitative level of Ki-67 could be used as a prognostic factor in breast-cancer patients.