Tamoxifen remains a frontline treatment for hormone-responsive breast cancer despite its use being associated with a 2–7-fold elevated risk of developing endometrial carcinoma. Several groups have investigated whether tamoxifen induces DNA-damaging (genotoxic) versus non-genotoxic mechanisms. Some studies point to the presence of tamoxifen-DNA adducts while others suggest otherwise. In many of these studies, the histological sub-type has not been considered; as type 1 carcinomas are associated with PTEN and KRAS2 mutations whereas type 2 carcinomas exhibit TP53 and ERBB-2 mutations, the absence of this information makes comparisons between such independent investigations difficult. An examination of the sub-types of endometrial carcinoma points to histological and mechanistic distinctions between sporadic and tamoxifen-associated disease; this could suggest differing aetiologies. On this basis, we propose a dual mechanism of action highlighted by the different patterns of endometrial carcinoma sub-types. Tamoxifen may initially be pro-oestrogenic in the endometrium giving rise to elevated type 1 endometrioid carcinoma occurrence whereas after long-term use, there is an increase of type 2 disease or malignant mixed mullerian tumours associated with a hormone-independent mechanism of action. Despite these associated risk factors, and the introduction of new selective oestrogen receptor modulators (SERMs), we suggest that the organ-specific pleiotrophic effects of tamoxifen mean that this effective therapeutic agent for breast cancer will continue to have significant usage. The focus of future research should concentrate on the different aetiologies of tamoxifen-associated endometrial carcinomas while efforts continue to develop future SERMs.