Background: Endometriosis is most commonly found in the peritoneum of the lesser pelvis and in the genital tract (in the ovaries). Its malignant transformation is quite rare, which usually appears in patients who previously underwent surgical procedures aimed at treating endometriosis. Years of hormone substitution (unopposed estrogen therapy) is also considered to have a role. According to the present authors' current knowledge, these are mostly well-differentiated tumors with low malignancy, which are primarily treated surgically. Case: In the present case the authors present a 73-year-old female patient who underwent a laparotomy due to abdominal pain and a mass in the lesser pelvis. The authors performed hysterectomy along with bilateral adnexectomy and omental resection. The histological examination of the specimens verified an endometrial adenocarcinoma formed on the ground of adenomyosis and the endometrial adenocarcinoma of the left ovary. Conclusion: The malignant transformation of endometriosis is rare, and the mechanisms how it develops on the grounds of adenomyosis is currently unclear.