Giant colonic diverticulum is a rare complication of diverticular disease of the colon and is thought to result, in most cases, from a "ball-valve" effect. The presentation and clinical course can be variable and confusing. The most common symptoms are abdominal pain and a palpable abdominal lump, with many patients presenting acutely with complications such as perforation and peritonitis. Preoperative diagnosis requires a high degree of suspicion and needs to be differentiated from sigmoid volvulus, caecal volvulus, intestinal duplication cyst, pneumatosis cystoidis intestinalis, and similar conditions. A plain x-ray and computed tomography (CT) scan of the abdomen shows a huge air-filled cyst termed "balloon sign" and confirms the diagnosis. The barium enema shows a communication with the bowel in most cases. In view of the high incidence of complications, treatment is advised even in asymptomatic cases and consists of excision of the cyst with resection of the adjacent colon with primary anastomosis. This treatment would, in most cases, be a sigmoid colectomy. Percutaneous drainage and Hartmann's procedure may be appropriate in some cases who present with a well-formed abscess or gross fecal peritonitis, respectively. A case is described, and the literature is reviewed.