Cribriform-morular variant of papillary carcinoma is a distinctive histological variant of thyroid cancer, characterized by intermingled cribriform, follicular, papillary, trabecular, and morular architecture. These tumors are known to be associated with familial adenomatous polyposis (FAP), but are also encountered in non-FAP patients. The authors report on ultrastructural and genetic studies of 3 patients with this type of carcinoma-associated FAP. There were numerous microfilaments approximately 100 nm long at the nuclear clearing area of the morular regions. Two of the 3 patients showed germline APC mutations, and 1 had so somatic APC mutation. Both mutations were in previously unreported regions. The study provides new information for understanding the development of this rare tumor.