This paper identifies another neoplasm of epithelial origin which may express vimentin in addition to cytokeratins, thereby adding to the expanding list of tumours which demonstrate intermediate filaments (IFs) other than those of their reputed cell of origin. Twenty examples of benign breast disease and 66 carcinomas were examined for vimentin and cytokeratin IFs using an avidin-biotin-peroxidase complex technique. Co-expression of these IF proteins was found in 35 per cent of cases of benign breast tissue and in 60 per cent of the carcinomas. In 8 (16 per cent) of 50 cases of infiltrating ductal carcinoma, vimentin and cytokeratin immunostaining was observed in more than 60 per cent of the tumour cells. These carcinomas were predominantly of a high histological grade. In benign breast disease and well-differentiated carcinoma, vimentin was distributed in the basal and perinuclear regions of the cells, with sparing of the apical portions. In those cases in which large numbers of tumour cells expressed vimentin, cytoplasmic staining was diffuse, and often exhibited distinctive perinuclear and subplasmalemmal accentuation. We propose that a knowledge of the list of carcinomas which may co-express vimentin and cytokeratin IFs might be helpful in the assessment of undifferentiated tumours and metastatic deposits.