A 77-year-old woman who had been examined 8 months previously because of chronic abdominal pain and an altered pattern of defecation presented to the emergency department with complaints of nausea, vomiting and acute pain in the abdomen. Her appetite was diminished and she had lost 10 kg in the past year. The abdominal X-ray showed a balloon-like, gas-filled intra-abdominal configuration, which proved to be a giant diverticulum of the sigmoid. She was treated by resection of the diverticulum and the sigmoid. A giant diverticulum is a rare complication of diverticulosis, a frequently occurring condition that is encountered most often in the sigmoid; the complication can easily be missed. The presenting symptoms can vary from an acute abdomen to chronic non-specific abdominal complaints. The most important complications of a giant diverticulum are perforation, obstruction or a volvulus. In view of the severity of these complications, resection of that part of the intestine in which the giant diverticulum arises is the treatment of choice.