OBJECTIVE To analyse the clinical and pathological features with long-term follow-up of a series of 12 cases of sarcomatoid carcinoma of the breast. methods and results: The cases were selected from the surgical files of the Department of Pathology, University of Edinburgh, between 1977 and 1988. The following clinical parameters were recorded: the age of the patients, size of tumour, presence or absence of lymph node or distant metastases, and patient survival. Pathological assessment included: the type of epithelial and mesenchymal components, the proportion of monophasic to biphasic tumours and the presence of adjacent in-situ carcinoma/atypical epithelial proliferation. The mean age of the patients was 61 years with a median of 64 and range 46-82 years. The mean size of the tumour was 52 mm (range 22-100 mm). None of the patients had distant metastasis at presentation and only one case had local lymph node metastasis which had a carcinomatous appearance. Five women were still alive after a minimum 12-year follow-up period. Four patients died of their disease (three with lung metastasis only and one with lung and bone metastases), one died of carcinoma of the cervix and two patients were lost to follow-up. Pathologically, four cases (33.3%) had no or almost undetectable epithelial structures by light microscopy, i.e. "monophasic sarcomatoid carcinoma". The remaining cases revealed varying proportions of both epithelial and mesenchymal elements, i.e. "biphasic sarcomatoid carcinoma". Of the epithelial component, six (50%) tumours had predominantly carcinoma of no special type, one lobular and one tubular carcinoma. The mesenchymal component was fibromatosis/nodular fasciitis-like, malignant fibrous histiocytoma-like (MFH), osteosarcoma-like and fibrosarcoma-like in five (42%), four (33%), two (17%) and one (8%) tumours, respectively. In 3/4 monophasic tumours, the mesenchymal component was of a low-grade fibromatosis/nodular fasciitis type. In 6/12 (50%) of the cases there was associated in-situ atypical epithelial proliferation (five ductal carcinoma in situ (DCIS) and one atypical ductal hyperplasia). CONCLUSIONS From this small series it appears that sarcomatoid carcinoma is an uncommon tumour, which is large in size and tends to lack local or distant metastasis at presentation. Pathologists should be alert to the presence of the bland monophasic sarcomatoid carcinoma which has a pure mesenchymal appearance on light microscopy, but epithelial components demonstrated by cytokeratin immunohistochemistry. These showed metastases on long-term follow-up, similar to other histological patterns of sarcomatoid carcinoma.