An infiltrating epithelial and spindle cell neoplasm developed in the breast of a 63-year-old female. An excisional biopsy was performed. Recurrence with rapid growth due to cyst development eventually resulted in more radical surgery. Interim fine needle aspirations had established its partially cystic nature. The unique microscopic appearance prompted the application of immunohistochemistry and electron microscopy. The tumour cells were found to exhibit characteristics denoting squamous and myoepithelial differentiation. Histopathological features of malignancy were absent. Our findings demonstrate the differentiation potential of breast epithelium. They are in concordance with the results of previous studies which delineate the histochemical and ultrastructural features of myoepithelial and establish the relationship of these cells to squamous metaplasia.