The authors present a case of a 16-year-old female diagnosed with rectal adenocarcinoma 10 years after receiving cranio-spinal radiotherapy for a cerebellar medulloblastoma. While the risk of a second malignancy is recognised to be increased in children previously treated with radiotherapy, rectal adenocarcinoma is a rare presentation. A child presenting with symptoms of weight loss and a change in bowel habit in a patient who has previously received radiotherapy should alert practitioners to the possibility of a colorectal malignancy.