While the association of endometrioma and ovarian cancer is recognized in epidemiological and pathological literature, the radiological appearances of this combined pathology have not been described. The purpose of this project was to write a descriptive paper presenting the cross-sectional imaging characteristics of ovarian cancer arising in association with endometrioma, diagnosed by MRI. Clinical and radiological findings of each patient as well as current literature regarding endometriomas and ovarian cancer were reviewed. The MRI findings in two cases of ovarian cancer arising in an endometrioma are presented. In both cases, the cancer was evident as an enhancing mass within a blood-filled adnexal cyst. We conclude that the MRI findings of an enhancing mass within a blood-filled adnexal cyst should prompt consideration of ovarian cancer arising in endometrioma; any suspected endometrioma should be carefully scrutinized for such an appearance to facilitate recognition of a superimposed malignancy and potentially allow for earlier and more efficacious treatment.