Ki67 immunostaining has been performed on 136 primary breast cancers and related to various clinical and pathological features of the disease. Staining was most frequently seen in poorly differentiated tumours showing high rates of mitotic activity, but was independent of tumour size, lymph-node status and ER expression. A high level of Ki67 immunostaining was often associated with early recurrence of breast cancer after mastectomy. These data are consistent with the concept of the Ki67 antibody detecting an antigen that is closely related to cell proliferation and thus provides a clinically useful marker for this important characteristic of the tumour.