BACKGROUND: Screening for colorectal cancer (CRC) has been shown to decrease mortality. OBJECTIVE: To examine determinants associated with having (1) a screening colonoscopy, (2) an appropriate indication for screening, and (3) a significant diagnosis at screening. DESIGN: Prospective observational study. SETTING: Twenty-one endoscopy centers from 11 countries. PATIENTS: Asymptomatic patients who underwent a colonoscopy for the purpose of detecting CRC and who did not have a history of polyps or CRC, a lesion observed at a recent barium enema or sigmoidoscopy, or a recent positive fecal occult blood test. INTERVENTION: Screening colonoscopy. MAIN OUTCOME MEASUREMENTS: Appropriateness according to the European Panel on the Appropriateness of Gastrointestinal Endoscopy (EPAGE) criteria and significant diagnoses (cancer, adenomatous polyps, new diagnoses of inflammatory bowel disease, angiodysplasia). RESULTS: Of 5069 colonoscopies, 561 (11%) were performed for screening purposes. Patients were more likely to have a screening colonoscopy if they were aged 45 to 54 years (odds ratio [OR] 2.53, 95% confidence interval [CI] 1.60-3.99). Screening colonoscopies were appropriate, uncertain, and inappropriate in 26%, 60%, and 14% of cases, respectively. Eighty-one significant diagnoses were made, including 4 cancers. Significant diagnoses were more often made for uncertain/appropriate indications (OR 3.20, 95% CI 1.12-9.17) than for inappropriate indications. LIMITATIONS: Although data completeness was asked of all centers, it is possible that not all consecutive patients were included. Participating centers were a convenience sample and thus may not be representative. CONCLUSIONS: About 1 of 10 colonoscopies were performed for screening, preferentially in middle-aged individuals. A higher diagnostic yield in uncertain/appropriate indications suggests that the use of appropriateness criteria may enhance the efficient use of colonoscopy for screening. [Authors]