This report describes a case of macroscopically visible cystic endosalpingiosis involving the paraovarian region in a woman who had been taking tamoxifen for breast cancer. A 2.5 cm multicystic lesion was seen on the external surface of the right ovary and histological examination showed a mass of dilated glands lined by ciliated tubal-type epithelium and set in a fibrovascular stroma. Cystic endosalpingiosis resulting in a tumour-like mass is a rarely described entity which is probably not well recognised by histopathologists. Although unlikely to be mistaken for malignancy, the lesion may result in diagnostic confusion. The role of tamoxifen in the development of the lesion in the present case is not clear but the oestrogenic effects of this drug may have contributed to its formation.