Although it is not uncommon to find multiple malignant neoplasms in the same organ, the incidence of synchronous malignant lymphoma and colonic adenocarcinoma has been very rare. This a case report involving a 52-year-old woman in whom malignant lymphoma actually arose within the same segment in which colonic adenocarcinoma was present. Under the diagnosis of advanced adenocarcinoma, she had been treated by right hemicolectomy. However, she developed intestinal obstruction about a month later. At the second surgery for this latter condition, cytologic examination of ascitic fluid revealed many malignant lymphoma cells. In spite of antilymphoma chemotherapy, the patient's condition became progressively worse during the subsequent five-month period, and she died of recurrent malignant lymphoma.