The proliferative marker, Ki-67, is a human nuclear antigen, and forms an integral part of cell division in both normal and malignant tissue. Since the hallmark of cancer is uncontrolled and relentless cell proliferation, the Ki-67 proliferative index is increasingly used to assess and manage breast cancer. The value of Ki-67 as a prognostic indicator, a guide to the selection of therapy, and a method of measuring response to ongoing treatment, is examined in this review.