The sessile serrated adenoma/polyp (SSA/P) has been thought as the relatively new precursor for the colorectal cancer. In the current review, the well-known pathological features including the histological definition of the SSA/P are described using the previous reports. Although the SSA/P is thought one of pre-cancerous lesions of the colorectal carcinoma, the decisive or documentary lesion like "carcinoma in adenoma" is very rare. In this review, the strict case of the carcinoma derived from the SSA/P is demonstrated using our cases. Although the genetic investigations of the SSA/P have shown the new pathway of colorectal carcinogenesis and these concepts are thought to be almost right, the verification for them should be performed using "the carcinoma in SSA/P" like the present case.