We report a case of rectal adenocarcinoma in a 9-year-old boy, which took the form of a second malignant neoplasm following treatment for an early childhood malignancy. The abdominal complaints were for a long time interpreted as an infectious disease. At the time of diagnosis of the rectal carcinoma, the tumor had already progressed to the stage of metastatic disease. Therapy consisted of deep anterior rectal resection and regional arterial chemotherapy for liver metastases. The child died 18 months after the diagnosis of rectal carcinoma. As survival for childhood tumors improves, rare second malignant neoplasms will become increasingly common in children and adolescents. This phenomenon emphasizes the need for continued clinical surveillance of patients who have been treated with chemotherapy or irradiation for childhood tumors. The increased risk of second malignant neoplasms and an early onset of adult-type tumors has to be considered.