In 250 patients with node-negative breast cancer and no systemic adjuvant therapy the impact on the prognosis of the variables age, histological tumour type, tumour size, histological grade, receptor status and localisation of the tumour within the breast, was studied. Patients were followed over a mean period of 60 (range 7-164) months. In a subset of 124 cases additional examination of the growth fraction rate, detected by immunohistochemical determination of antibody Ki-67 and semiquantitative measurement of the stained tumour cell nuclei was performed. In 43 cases, measurement of S-phase fraction by flow cytometry was also performed. By univariate analysis, the histological tumour type (ductal/non-ductal), histological grade and growth fraction rate (< = 20%/> 20% tumour cell nuclei stained by antibody Ki-67) were found to exert a significant influence on distant disease-free and overall survival. Cox' multivariate regression exhibited histological tumour type and growth fraction rate to be independent predictors of distant disease-free and overall survival. Additionally, age was found to be an independent variable of distant disease-free survival. Measurement of growth fraction rate by immunohistochemical detection of Ki-67 antigen is a fairly simple and easily applicable procedure. It should be discussed whether a growth fraction rate of more than 20% could be an indication for adjuvant chemotherapy in patients with node-negative breast cancer.