We used KPNC electronic medical records to select patients who had undergone colonoscopies between 1 January 1994 and 16 July 2002. This was an observational study, conducted in medical centers throughout the KP health care system, evaluating practice patterns as they existed at the time the included colonoscopies were performed by the endoscopists in the study. Electronic records were reviewed to identify immediate complications, outpatient visits, or hospital admission within 30 days of colonoscopy. Colonoscopies were included in the analysis if they were performed for patients 40 years of age or older who were undergoing coloscopy because of a family history of colorectal cancer or adenomatous polyp, as a follow-up to a positive screening test (that is, polyp or cancer at sigmoidoscopy, positive results on a fecal occult blood test, or abnormal barium enema radiography), for surveillance because of a previously detected adenomatous polyp or colorectal cancer, or for primary screening. Colonoscopies were not included if the procedure was being performed to diagnose symptoms (for example, diarrhea, abdominal pain, gastrointestinal bleeding, history of rectal bleeding, or anemia) or if patients had outpatient visits 6 months before the procedure for abdominal pain, anemia, diarrhea, or constipation.