Background and Aims: Evidence from Japanese studies suggests that nonpolypoid colorectal neoplasia (NP-CRN) tends to be more pathologically advanced than polypoid neoplasia. However, data are limited regarding the prevalence of NP-CRN in an average-risk population. In addition, the diagnostic yield of the fecal occult blood test (FOBT) in relation to different types of colorectal neoplasms remains unclear. We prospectively investigated the prevalence and characteristics of polypoid and nonpolypoid colorectal lesions in an asymptomatic and average-risk Chinese population. Methods: The study included 12,731 asymptomatic Chinese subjects (8372 of whom were average-risk subjects) who under-went screening colonoscopy. The prevalence, histopathologic findings, and topographic distribution of polypoid and nonpolypoid colorectal lesions were determined and analyzed. The diagnostic yield of FOBT, in relation to lesion morphology, also was assessed. Results: NP-CRN was detected in 552 (4.3%) asymptomatic and 348 (4.2%) average- risk subjects. The prevalence of depressed NP-CRN was 0.18% in both asymptomatic and average-risk subjects. A higher proportion of smaller-sized but high-grade dysplasia and invasive carcinoma beyond the submucosal layer was noted for depressed NP-CRN compared with flat NP-CRN or polypoid neoplasia. The diagnostic yield of FOBT was comparable in depressed lesions and their polypoid counterparts. Conclusion : The prevalence of NP-CRN is substantial in both asymptomatic and average-risk Chinese individuals. Some subcategories of NP-CRN in this population tend to have more advanced pathologic characteristics. These findings may lead to modification of screening and prevention strategies for colorectal cancer. ; 附設醫院內科部 ; 醫學院附設醫院 ; 期刊論文