Colonoscopy has established benefits for the detection and prevention of colorectal cancer, which is the second leading cause of cancer-related deaths in Canada and the United States. However, two recently published studies from scientists at the Institute for Clinical Evaluative Sciences (ICES) have found that the procedure has certain limitations (Baxter et al. 2009; Rabeneck et al. 2008). First, colonoscopy seems to be less effec-tive at preventing cancer deaths from tumours that originate in the right side of the large bowel. Second, while the procedure is safe for many patients, certain people appear to be at increased risk for serious complications. Colonoscopy is widely used to detect both colorectal cancer and adenomatous polyps, which may become malignant if left alone. During a complete colonoscopy, a physician - usually a gastroenterologist or general surgeon - inserts a long, flexible tube called a colonoscope through the patient's rectum and along the length of the large bowel. The goal is to scan the entire colon for potentially cancerous or pre-cancerous growths. If such a polyp or lesion is detected, it can often be removed during the colonoscopy so that no additional procedures or surgery are needed. The findings from the two new ICES studies, detailed below, are especially important given the widespread and increasing use of colonoscopy and the need to evaluate possible harms from and limitations of the procedure.