['We investigated the rate and predictors of anesthesia assistance during outpatient colonoscopy and whether anesthesia assistance is associated with colonoscopy interventions and outcomes.', 'We performed a retrospective cohort study using a 20% sample of Medicare administrative claims submitted during the 2003 calendar year. We analyzed data from 328,177 adults, 66 years old or older, who underwent outpatient colonoscopy examinations.', 'Overall, 8.7% of outpatient colonoscopies were performed with anesthesia assistance. In multivariate analysis, independent predictors of anesthesia assistance included black race, female sex, and a nonscreening indication; anesthesia assistance increased with median income and comorbidities. General and colorectal surgeons, fewer years in their practice, and nonhospital site of service were also significantly associated with anesthesia assistance. The strongest predictor of anesthesia assistance was the Medicare carrier, with odds ratios ranging from 0.22 (95% confidence interval: 0.12-0.43) for the Arkansas carrier (crude rate 0.9%) to 9.90 (95% confidence interval: 7.92-12.39) for the Empire carrier in New York area (crude rate 35.3%) compared with the Wisconsin carrier (crude rate 4.3%). There was also considerable variation among endoscopists; 75% of providers had no colonoscopies with anesthesia assistance recorded in their dataset, and 4.5% of providers had anesthesia assistance in at least three quarters of their examinations. Anesthesia assistance was not associated with the diagnosis of polyps, the performance of biopsy or polypectomy, or complications in multivariate analyses.'] Is regional variation in anesthesia assistance during outpatient colonoscopy associated with differences in polyp detection or complication rates?