The pathogenesis and risk of malignancy of traditional serrated adenomas (TSAs) are unclear. In North America, TSAs are relatively uncommon, occur mainly in the left colon, and in some studies, have not been shown to have a strong association with hyperplastic polyp (HPP) or sessile polyp adenoma (SSA) precursor lesions. In the Far East, and particularly in Korea, TSAs are more common and occur both in the left and right colon. However, the pathogenesis of TSAs in Korean patients, and the similarity to those that occur in North America, have never been evaluated. The purpose of this study was to determine the frequency and type of precursor lesion in TSAs, and to determine the molecular profile according to the grade of histologic dysplasia and/or cancer and anatomic location of the colon in a cohort of Korean patients. One hundred and twelve TSAs were evaluated pathologically and categorized according to the grade of dysplasia (either low or high grade) and the presence or absence of adenocarcinoma. TSAs were also separated into those with serrated versus conventional adenomatous dysplasia. As controls 35 conventional adenomas were evaluated, 14 of which had adenocarcinoma. All lesions were evaluated for the presence and type of precursor lesions and for KRAS and BRAF mutations and methylation of MGMT, hMLH1, and APC. A nondysplastic precursor lesion (HPP or SSA) was identified in 35 TSAs (31.3%). TSAs with a precursor lesion were more commonly found in the right colon compared with the left colon (P=0.03). Mutations of KRAS and BRAF and methylation of MGMT, hMLH1, and APC were present in 29%, 55%, 63%, 56%, and 37% of TSAs, respectively. TSAs with high-grade dysplasia and intramucosal adenocarcinoma showed a significantly higher frequency of KRAS mutation and MGMT methylation, and a significantly lower frequency of BRAF mutations, compared with TSAs with low-grade dysplasia (P<0.05). KRAS mutations were more prevalent in TSAs from the left colon and correlated significantly with higher grades of dysplasia. In a subgroup of TSAs in which both the precursor and neoplastic components were evaluated, a similar molecular profile was shown in both types of epithelium. Our results suggest that up to one-third of TSAs show a histologically identifiable nondysplastic HPP or SSA precursor lesion, particularly in lesions from the right colon. The development of KRAS mutations and methylation of MGMT may herald the onset of an aggressive phenotype in the neoplastic progression of TSAs and also suggests that a fusion between the serrated pathway of carcinogenesis and the chromosomal instability pathway may occur in some TSAs. Further studies are needed to determine the natural history and risk of malignancy of TSAs, specifically related to the anatomic site of development.