A mother and son were found to have multiple juvenile polyps of the colon; later a metastatic adenocarcinoma of the large bowel developed in the mother. In both patients, histologic examination of the polyps did not reveal adenomas, but some of the juvenile polyps contained adenomatous-like elements. Because of the mother's history of colon cancer and the adenomatous features in some of the son's juvenile polyps, he underwent an elective subtotal colectomy. This family may represent a phenotype variation with features of both the juvenile and adenomatous polyposis syndromes and the presence of dysplasia of the glandular epithelium could be a marker of potential malignancy in patients with juvenile polyposis coli.