A case of long diverticular colonic duplication producing acute abdominal pain in a 6-year-old girl is presented. Physical examination showed no signs of acute abdomen at the initial presentation. After a pain-free interval, there was a sudden onset of severe abdominal pain and a large tumor in the lower abdomen was observed. A plain x-ray showed an enormously dilated colonic pouch filled with gas. Excision of the T-shaped duplication and small part of the transverse colon was successful. Because of extensive fibrotic changes in the colon near the opening of duplication, a resection margin of at least 2 cm is recommended.