Immunoreactivity of the monoclonal antibody Ki67, which recognizes an antigen expressed in cells active in the cell cycle, has been investigated by immunocytochemistry in a series of 67 primary breast cancers. The percentage of tumour cell nuclei stained by Ki67 (labelling index) was related to tumour histological grade, mitotic frequency, oestrogen receptor status and tumour type. No correlation was found with patient age, tumour size or lymph node stage. A high Ki67 labelling index was significantly associated with diminished patient survival and disease-free interval, which demonstrates an important role for this monoclonal antibody as a prognostic marker in breast cancer.