Introduction:
Malignant transformation of endometriosis is a rare entity. Its occurrence in the digestive system is exceptional. In fact, due to the atypical symptoms, the diagnosis is often delayed at an advanced stage. So, the treatment strategy should be discussed in a multidisciplinary meeting.

Aim:
Our objective was to describe a challenging diagnosis of an in situ carcinoma developed on a cystic foci of endometriosis of the colon in a woman without a particular past medical history.

Case study:
A 32-year-old female patient presented with abdominal pain for about six months. The different explorations have concluded to an ovarian cystic tumour. Intraoperatively, the mass seemed to be developped in the right colon. This led to perform a right hemicolectomy. Gross findings consisted in a sub-mucosal 10-centimeters cyst and microscopic features were consistent with an in situ carcinoma developed on a cystic endometriosis foci.

Discussion:
This case illustrates the malignant potential of endometriosis especially when it is misdiagnosed.

Conclusions:
Besides the fact that this case was illustrated by radiological and microsocpic features, it puts emphasis on the non consensual management of a rare lesion of the colon.