The differential diagnosis of lower gastrointestinal bleeding in children can be reduced markedly simply by taking into account the age of the child. The clinical condition of the patient can further help narrow the diagnostic possibilities. Newborns and infants who are clinically unstable are more likely to have diseases such as necrotizing enterocolitis, volvulus, Hirschprung disease, intussusception, or Meckel diverticulum. A baby who appears healthy should be examined for swallowed blood, allergic colitis, anal fissures, or lymphonodular hyperplasia. An older child of healthy appearance with bleeding is likely to have a juvenile polyp or infectious colitis, but a child who appears sick may have hemolytic uremic syndrome, Henoch-Schoenlein purpura, or inflammatory bowel disease. This information, along with that gleaned from the physical examination, can lead the pediatrician to determine the need for specific tests, such as abdominal radiographs, stool cultures, and an endoscopic evaluation. We have come a long way in our ability to diagnose the causes of lower gastrointestinal bleeding. With the availability of newer radiographic and nuclear medicine modalities and the ability to visualize the colon endoscopically, the need for exploratory laparotomy for diagnosis is rarer. While surgery may still be the therapy of choice, new diagnostic modalities give the surgeon much more preoperative information.