Uterine malignant mixed Mullerian tumour is a very aggressive tumour of unknown aetiology. Previous pelvic irradiation, excessive oestrogen exposure, obesity and nulliparity have been incriminated in its aetiology. There is a paucity of data on the association between uterine mixed Mullerian tumour breast cancer and prolonged tamoxifen treatment. We report four patients who took tamoxifen for more than 7 years for breast cancer prior to their diagnosis with uterine mixed Mullerian tumour and also reviewed similar cases reported in the medical literature. The purpose of this article is to draw the attention of other professionals on this subject.