A 54-year-old woman who underwent a hysterectomy and radiation therapy for carcinoma of the uterine cervix developed mucosal de novo cancer of the rectum 12 years later. The rectal lesion was elevated, measuring 0.8 x 0.7 x 0.3 cm, and its surface was relatively smooth. Microscopically, this tumor consisted of a well-differentiated tubular adenocarcinoma which had invaded the depth of the mucosa. Late radiation injuries were observed around the tumor. We diagnosed this lesion as a radiation-induced mucosal de novo adenocarcinoma of the rectum on the basis of the criteria of Black and Ackerman (Clin Radiol 16:278, 1965). This case underscores the need for careful long-term follow-up studies of the large intestines of patients who underwent therapeutic irradiation for uterine cancer.