Familial polyposis is a rare but well documented hereditary disease characterized by the development in the colon and rectum of large numbers of adenomatous tumors. Other names which have been applied to this entity are familial adenomatosis, diffuse polypoid disease, multiple adenomata or papillomata, and hereditary multiple polyps, but the term now most frequently employed is familial intestinal polyposis. Although numerous articles have stressed the genetic, surgical, and pathologic features of this interesting disease, there have been infrequent references to the problems of roentgen diagnosis. It is the purpose of this report to present the roentgen features, including differential diagnosis, in 20 patients. The lesions usually appear in childhood or early adult life as tiny nodules, later becoming large sessile or pedunculated polyps, as a result of extensive proliferation of the glandular epithelium. After months or years, malignant change almost always occurs in one or more of the adenomas. The precancerous nature of the disease was first recognized by Lockhart-Mummery in 1925 (10). McKenney divides familial polyposis into two types, the adult or acquired type and the adolescent, congenital, or disseminated type. His conclusions are based mainly on an investigation of two families (11, 12). After studying the relationship between the size of the polyps and the age of the patient, he concluded that these lesions were most likely present at birth, though they probably measured no more than 1 mm. in diameter at that time, making diagnosis almost impossible. He suggested that a more intensive study of infants and young children in known polyposis families might give more reliable information regarding the age of onset of this disorder. Numerous workers have studied the genetics of this disease, notably Gardner (6) and Dukes (4), and have stressed the strong familial tendency. Familial polyposis is transmitted to males and females alike, and in most of the families about 50 per cent of the children inherit the disease in line with the mendelian laws of inheritance of a dominant gene. Dukes (4, 5) has followed the clinical history of a large number of such families. His main conclusions are that the average age of onset is twenty years; that symptoms rarely develop for the first time after forty years, and that cancer is most likely to be discovered about fifteen years after the first symptoms. In Dukes' patients, the average age at death was approximately twenty-six years younger than for the general population, and of 156 cases of polyposis, 114 were known to have become malignant. Although the disease is usually familial, isolated cases can occur. Whether there is any essential difference between these two forms of the disease has not been established. Dukes (4) found that in 11 of his 33 cases, only one member of the family was affected.