A 64-year-old woman presented with severe abdominal pain and was found to have a large fecolith in the sigmoid colon with resulting bowel obstruction. During a therapeutic colonoscopy, she developed severe shortness of breath and hypoxia, and was found to have a tension pneumothorax. We review the potential mechanisms by which pneumothorax may occur following colonoscopy. In addition, the eight previously published cases are reviewed. Pneumothorax, with or without pneumomediastinum, can occur through a variety of mechanisms following colonoscopy. Although rarely reported, this may represent an underappreciated complication and should be fully investigated in the appropriate setting. Colonoscopy, an exceedingly common procedure, will continue to increase with the aging population. As a result, tension pneumothorax can have a profound effect on the patient outcome and therefore physicians, both gastroenterologists and pulmonologists, should be aware of all the potential problems with this procedure.