We report a case of gigantic cystic lymphangioma of the ascending colon excised through an open laparotomy. A 34-year-old woman consulted a gynecologist for treatment of infertility. Transvaginal ultrasonography revealed a cystic mass in the pelvis, and she was transferred to our hospital for further investigation. Abdominal enhanced computed tomography (CT) showed a bulky cystic mass, 25 cm or larger, in the abdominal and pelvic cavity. Colonoscopy revealed a cystic submucosal tumor with a cushion sign. Cystic lymphangioma was diagnosed and excised via open surgery as we could not exclude its malignant potential. Pathological examination confirmed lymphangioma. To our knowledge, this is the most gigantic lymphangioma of the colon documented in the literature. About 3 months after surgery, the patient discovered that she was pregnant and her first baby was delivered at term, uneventfully.