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We reviewed the clinicopathological findings of eight cases of sessile serrated adenoma/polyps (SSA/Ps) with carcinoma, the largest diameter of which was 10 mm or less. All lesions were polyps located in the right side of the colon. Four lesions showed submucosal invasion and one lesion invaded the proper muscle layer. The depth of invasion, however, did not seem to be related to the carcinoma area size. Most carcinomas were well to moderately differentiated tubular adenocarcinomas focally showing some serrated appearances, and the predominant component of one carcinoma was a poorly differentiated medullary growth with inflammatory stroma. Rapid progression to invasive carcinoma from SSA/P was suggested for the carcinoma with proper muscle invasion whereas one submucosally invasive carcinoma was considered to progress over 7 years. Immunohistochemically, it was suggested that with or without hMLH1 protein loss, alterations of p53 and/or Wnt signaling pathway can be involved in the cancerization through SSA/Ps. The carcinomas irregularly imitated the mucin expression of the SSA/Ps (positive for MUC5AC and MUC2, and MUC6 expression in crypt bases), which was lost with progression of the carcinomas. Analyses of small SSA/P lesions with cancerization would facilitate the understanding of the mode of progression of SSA/Ps and their early detection.