Five patients with giant colonic diverticulum (GCD) are reported. Three of them had complications of the GCD including free perforation with pneumoperitoneum and peritonitis and an instance of carcinoma arising in a GCD. The preoperative diagnosis of GCD is made radiographically with findings of a large, smoothly marginated, round or oval, homogenous radiolucency in the abdomen that is in close apposition to the colon on barium enema examination. Barium enters the GCD in 60% of the cases. Most GCDs are considered to be pseudodiverticula; however, an occasional case may have all layers of bowel in the wall of the diverticulum and is believed to represent a true diverticulum or a communicating partial colonic duplication.