Flexible colonoscopy is commonly performed in dogs with signs of large-bowel diseases. Although considered to be a safe procedure, no reports of complications associated with colonoscopy have appeared in the veterinary literature. The purpose of this study was to describe the frequency and types of adverse events that developed during flexible colonoscopy in dogs. Medical records were reviewed from 355 scheduled colonoscopic procedures. Major complications were defined as adverse events in which the dog's life was potentially jeopardized and the complication required intensive treatment or monitoring. Major complications consisting of fatal aspiration of GoLYTELY, colonic perforation, and excessive hemorrhage after biopsy of an adenocarcinoma with rigid forceps occurred in 3 (0.85%) dogs. Minor complications associated with anesthesia or colonoscopy occurred during 3.4% of procedures. Complications were classified as minor if the adverse event required minimal treatment or monitoring, and the complication was not considered a threat to the dog's life. Vomiting of GoLYTELY occurred with the administration of 4.6% of doses in 6.5% of dogs. When administering GoLYTELY, clinicians should be prepared to rapidly remove the orogastric tube and mouth speculum if vomiting occurs to reduce the potential for aspiration. In this group of dogs undergoing flexible colonoscopy, major complications occurred infrequently and minor complications developed uncommonly. Overall, minor or major complications developed during 30 (8.5%) of 355 procedures. Mortality was rare (0.28%). Flexible colonoscopy appears to be a safe procedure in dogs with signs of large-bowel diseases.