A 34-year-old patient was scheduled for valve replacement to treat a symptomatic mitral regurgitation. The preoperative work-up incidentally discovered an intra-abdominal cystic tumour extending from the epigastrium to the pelvic region on a computed tomography scan. The patient had no abdominal symptoms by the giant cyst from unkown origin. An open “en bloc” resection disclosed a large cyst in the mesocolon. Pathological examination, including immunohistochemistry, enabled the diagnosis of a mesenteric cystic lymphangioma. Long-term follow-up of 12 months shows no recurrence. Mesenteric cystic lymphangioma, which is extremely rare in adults, is a challenge to diagnose and needs complete resection to ensure dignity and to avoid recurrence.