Background and Study Aims: The factors that more accurately predict the detection of colorectal cancers and adenomas at colonoscopy are different. We conducted a prospective multicenter study to evaluate which indications were most closely associated with advanced colorectal neoplasm (CRN), including colorectal cancer, in a group of patients undergoing colonoscopy. Patients and Methods: The 17468 patients were enrolled in this study between July 2003 and March 2004, from 11 tertiary medical centers in Korea. They were recruited according to 11 itemized colonoscopic indications. The term "advanced adenoma" refers here to tubular adenomas of diameter of 11 mm or more, or to tubulovillous, villous, or severely dysplastic adenomas, irrespective of their size. Cancer was defined as the invasion of malignant cells beyond the muscularis mucosa. Advanced CRN was defined as advanced adenoma or invasive cancer. Results: Advanced CRN was found in 1227/17 307 patients (1176 advanced adenomas plus 51 carcinomas, 7.1%). According to univariate and multivariate analysis, the factors associated with advanced CRN included age >60 years (odds ratio (OR) 2.1, 95% confidence interval (CI) 1.8-2.4, P < 0.0001), male gender (OR 2.1, 95%CI 1.7-2.7, P < 0.0001), referral for colonoscopy from primary care physician (OR 3.1, 95%CI 2.5-3.7, P < 0.0001), and several other indications (OR 1.8, 95%CI 1.5-2.3, P < 0.001). The yield of colonoscopy for advanced CRN was lower (2.2%) than expected in patients with iron-deficiency anemia (OR 0.5, 95%CI 0.2-0.9, P=0.03). Conclusions: Age, gender, and referral for colonoscopy from primary care physician constituted important independent predictors of advanced CRN in patients undergoing colonoscopy.