Endometriosis is a chronic disease characterised by the presence of endometrial tissue outside the uterine cavity, affecting 5–15% of women, especially those of reproductive age. The disease may manifest itself as dysmenorrhoea, dyspareunia, sterility and chronic pelvic pain, among other symptoms. Although it is not malignant, it shares some characteristics with cancer and can lead to epithelial ovarian carcinoma. The risk of malignant transformation of endometriosis is estimated at 1% in premenopausal women and 1–2.5% in postmenopausal women. Our case report describes a 46-year-old female patient with long-standing abdominal pain and a history of surgically confirmed endometriosis. Imaging revealed a cystic mass in the left mesogastrium, which was subsequently surgically removed. Histological examination confirmed the presence of a low-grade endometrioid carcinoma arising from an extragenital endometriosis lesion. Following surgical treatment, the patient underwent adjuvant chemotherapy, after which she was in complete remission. The diagnosis of malignant transformation of endometriosis is complex, requiring a combination of thorough clinical examination, imaging, and histopathological verification. Therapy involves radical surgery and possibly adjuvant chemotherapy, similar to ovarian carcinomas. Despite advances in treatment and research, endometriosis remains a complex disease with unclear aetiology, heterogeneous clinical presentation, and risk of malignant transformation. Key words: endometriosis– malignant transformation – extragenital lesion – diagnosis – treatment