A 30-month-old twin with Beckwith-Wiedemann syndrome underwent exstirpation of a nephroblastoma from the right kidney. A Meckel's diverticulum (MD), an incidental finding intraoperatively, was resected because it was indurated and enlarged. Histologically, the MD contained ectopic gastric and pancreatic mucosa. Nesidioblastosis was identified within the pancreatic tissue. This is probably the first description of ectopic nesidioblastosis within a MD.