Tamoxifen treatment is a proven therapy for breast cancer that produces a survival advantage when used as an adjuvant, and reduces the incidence of recurrences and controlateral tumor evolution. Although this therapy has a very low toxicity profile, an increase in secondary cancers has been reported. Tamoxifen is suggested to be carcinogenic both through direct genotoxic and epigenetic mechanisms. It is activated by cytochrome P450 to form reactive metabolites that bind covalently to DNA to create adducts. We report two cases that developed acute myeloid leukaemia (AML) during tamoxifen therapy for breast cancer.