The proband was a 49-year-old woman who had undergone total colectomy, ileorectostomy, and bilateral ovariectomy for the treatment of cecal(T3N0)and sigmoid colon(T4a, N2b, M1c2[Ova], Stage Ⅳc)cancers. Pathological findings revealed 6 adenomas and 2 adenocarcinoma-in-adenomas in the right colon, other than advanced colon cancers. She had a family history of colorectal cancer meeting the Amsterdam Criteria I, but none of her relatives had definite polyposis. Considering the possibility of Lynch syndrome, the microsatellite-instability test and immunohistochemistry(IHC)examination of the mismatch repair protein were performed, leading to the results of microsatellite stable and proficient mismatch repair protein expression. Therefore, we performed the multigene panel test containing 26 genes using the next-generation sequencing technology. In the APC(5q22.2)gene, a pathogenic variant(exon 12 c.994C>T/p.Arg332*)was identified, leading to a diagnosis of attenuated familial adenomatous polyposis(AFAP). After disclosure of the results to the proband, the single-site variant analysis was performed on her 3 daughters. In her second and third daughters, the same variant was confirmed, and laparoscopic total colectomy was performed 23 and 35 months after the disclosure of the genetic analysis results, respectively. Currently, we are conducting periodical surveillance for the residual rectum.