This report describes a positive relationship between vimentin expression in infiltrating ductal breast carcinoma, and high tumour growth fraction. Vimentin expression is potentially a predictor of aggressive behaviour, and such carcinomas may benefit from early adjuvant therapy. Eighty-four malignant breast neoplasms were stained with monoclonal anti-vimentin and anti-cytokeratin antibodies. The tumour growth fractions were determined by immunostaining cryostat sections with the Ki-67 antibody. Seven (9.2 per cent) of 76 infiltrating ductal carcinomas co-expressed cytokeratin and vimentin intermediate filaments in more than 50 per cent of neoplastic cells. In each case, the corresponding Ki-67 count was much greater than 40 per cent, significantly higher than the mean growth fraction for all tumours examined (P less than 0.0001). Vimentin immunoreactivity was also positively related to the histological grade of the ductal carcinomas (P less than 0.002) and inversely related to tumour ER count (P less than 0.0002) and patient age (P less than 0.01). No relationship was observed between vimentin positivity and either the presence of axillary nodal metastases or primary tumour size.