A case of malignant myoepithelioma of the breast, associated with in situ and invasive carcinoma NOS is described. Myoepithelial differentiation was demonstrated with immunohistochemistry and electron microscopy. The tumour affected the left breast of a 72 year old lady. The patient had been treated with quadrantectomy with axillary dissection, followed by radiotherapy. At the time of diagnosis no local or distant metastases were found. Bone, pulmonary and cerebral metastases appeared 28 months after treatment. Malignant myoepitheliomas share histological and immunohistochemical features with monophasic sarcomatoid carcinomas. Comparison and and possible relationship with monophasic sarcomatoid carcinomas is discussed.