Introduction:Polypoid endometriosis is a rare variant of endometriosis characterized by histological features resembling an endometrial polyp. Lesions frequently affect the ovaries presenting as adnexal masses which may mimic malignancy, with an extremely complex differential diagnosis due to the poor evidence reported in Literature.Case description:In this report, we describe the case of a 43 years old woman referred to pelvic transvaginal ultrasound examination for recurrent abdominal pain, in whom sonography revealed a pelvic mass with features highly suspicious for ovarian carcinoma. Surgical removal allowed histopathological diagnosis of polypoid endometriosis, with no signs of malignancy. At ultrasound examination, the lesion appeared as a multilocular-solid mass, with low-level echogenicity of cystic content, multiple papillary projections, and solid areas with high vascularization and apparent infiltration of the uterus. At retrospective review of the sonographic images after pathological examination, some features mirroring the histological architecture of polypoid endometriosis could be identified in the solid components of the mass: these included the hyperechoic appearance, the rounded outline of the intracystic projections, and the vascularization pattern with a single central vessel with branching.Conclusions:To date, this is the first work providing a detailed ultrasonographic description of polypoid endometriosis using shared terms and definitions and relating these findings with available evidence about radiologic and histopathologic features. The report shows how this condition could strongly mimick ovarian malignancy, though several sonographic features can be identified reflecting the histopathological patterns of those lesions.