BACKGROUND: Colorectal cancer incidence is rapidly rising in many Asian countries, with rates approaching those of Western countries. This study aimed to evaluate the prevalence and trends of colorectal adenomas by age, sex, and risk strata in asymptomatic Koreans. METHODS: Cross-sectional study of 19,372 consecutive participants aged 20 to 79 years undergoing screening colonoscopy at the Center for Health Promotion of the Samsung Medical Center in Korea from January 2006 to June 2009. RESULTS: Among participants at average risk, those without a history of colorectal polyps or a family history of colorectal cancer, the prevalence of colorectal adenomas and advanced adenomas were 34.5% and 3.1%, respectively, in men and 20.0% and 1.6%, respectively, in women. The prevalence of adenomas increased with age in both men and women, with a more marked increase for advanced adenoma. Participants with a family history of colorectal cancer or with a history of colorectal polyps had significantly higher prevalence of adenomas compared with participants of average risk (36.9% vs. 26.9%; age- and sex-adjusted prevalence ratio = 1.16; 95% confidence interval, 1.09-1.22). The prevalence of adenomas increased annually in both men and women. CONCLUSIONS: In this large study of asymptomatic Korean men and women participating in a colonoscopy screening program, the prevalence of colorectal adenomas was comparable and possibly higher than previously reported in Western countries. IMPACT: Cost-effectiveness studies investigating the optimal age for starting colonoscopy screening and etiological studies to identify the reasons for the increasing trend in colorectal adenomas in Koreans are needed.