Familial juvenile polyposis is a rare intestinal polyposis that has recently been associated with gastric and colonic adenocarcinoma. The authors report a kindred of 41 members, 11 of whom have familial juvenile polyposis. In these patients, random sections of otherwise grossly normal-appearing colonic mucosa showed a dense population of mixed inflammatory cell infiltrates in the superficial third of the lamina propria. Fine nodular mucosa was noted focally and diffusely in six of eight colons resected. These consisted of foci of dense inflammatory cell infiltrates in the mucosa with slight crypt architectural abnormalities. The majority of lesions were typical juvenile polyps. Dysplastic changes were noted in the polyps that were 1-2.9 cm or larger. The largest polyps contained foci of villous adenoma and juvenile polyp. A focus of adenocarcinoma of the colon was noted at the base of the villous adenoma portion of the largest polyp. The gastric polyps were histologically identical to hyperplastic polyps of the stomach. This report represents the largest number of patients (eight) in a single family with familial juvenile polyposis studied histologically. This is also the first time that the changes in the nonpolypoid colonic and gastric mucosa have been reported. The pattern of inheritance in this family suggests that the trait for familial juvenile polyposis segregates as an autosomal dominant.