Tamoxifen therapy is associated with an increased risk of endometrial cancer. There is controversy regarding the incidence of high-grade endometrial malignancies associated with tamoxifen therapy. This retrospective study assesses pathological features of endometrial malignancy in patients with and without a history of tamoxifen therapy. This is a retrospective case analysis from a district general hospital. The cases were identified from the pathology database and the medical notes reviewed. The period of the study was January 1994 to December 2001. One hundred and ninety six women with endometrial cancer were identified. 20 patients had a history of breast cancer being treated with adjuvant tamoxifen therapy and developed subsequently endometrial cancer. The histology in women who had not taken tamoxifen showed: adenocarcinoma (97.1%), 1.7% had mixed mullerian tumour. Women in the tamoxifen-treated group had: adenocarcinoma (85%), sarcoma (5%), mixed mullerian tumour (5%). In this study the tamoxifen-treated group of patients developed endometrial malignancies with a higher incidence of poor prognostic malignancies (p = 0.01). Further research is needed to analyse the precise tumour types and pathophysiology.