Background: Colonoscopy, introduced in the late 1960s, has become the principal method for diagnosis, treatment, and follow-up of colorectal diseases. Being invasive, colonoscopy is associated with a risk of complications. The aim of this study was to analyze the rate of complications of diagnostic and therapeutic colonoscopy in a population-based setting. Methods: All colonoscopy records for 1979 to 1995 in 1 Swedish county (population 258,000) were retrieved. Information was obtained about patients' demographics, date of examination, endoscopist, indications, findings, colonoscopy type, completion level, and complications. Records were linked to the Cause of Death Register and the Swedish Hospital Discharge Register to ascertain mortality and morbidity. Results: In 6066 colonoscopies, the overall morbidity was 0.4% (diagnostic 0.2%, therapeutic 1.2%). The most frequent complications were bleeding (0.2%) and perforation (0.1%), with no colonoscopy-related mortality. Bleeding was confined to therapeutic colonoscopy and occurred immediately, mainly after removal of large polyps with thick stalks. Perforations at diagnostic colonoscopy occurred in the left colon; they were diagnosed sooner than perforations associated with therapeutic colonoscopy where the cecum was the most frequent site. The bleeding rate was correlated to the experience of the endoscopists. Conclusions: Colonoscopy is a safe procedure, and the rate of adverse events in this population-based setting was low. (Gastrointest Endosc 2001;54:302-9.)