A leiomyoma-like, multifocal tumour developed from intraductal papillomatosis in the breast of a 42 year old woman. The spindle-shaped tumour cells were examined by light, electron and polarizing microscopy, which revealed that they originated from immature "precursor" myoepithelial cells. The author suggests that the tumour be called "myoepithelioma". From the morphological characteristics of the tumour the myoepithelial cells appear to be capable of producing leiomyoma-like benign or malignant tumours. The role that has been attributed by some to the myoepithelial cell in the production of epithelial tumours is problematical, in the light of the present finding.