Mammary myofibroblastoma is a rare tumour, mainly occurring in male patients. This paper describes two cases of mammary myofibroblastoma, with typical histological features, including the presence of fat cells and mast cells. Immunohistochemically, the spindle cells stained positively for desmin and CD34. Cytogenetically, both tumours showed partial monosomy 13q and in case 1, there was, in addition, partial monosomy 16q. Rearrangements affecting 13q and 16q occur typically in spindle cell lipomas. In addition to histological similarities, the hitherto unreported chromosomal changes in mammary myofibroblastoma, which are similar to the chromosomal aberrations in spindle cell lipoma, strongly suggest a link between these two tumours and are not in favour of myofibroblastoma being a primary solitary fibrous tumour of the breast.