The diagnosis of familial polyposis depends on there being more than 100 adenomatous polyps in the large bowel. The polyps are the result of intramucosal microadenomatous growth. The age at which this occurs varies, and in the early stages of polyp development relatively few larger polyps may be seen. The numbers and size of the polyps as seen on double-contrast barium enema were compared with the macroscopic findings on the resected specimens in 27 patients with proven polyposis. Of these patients, 23 (83%) were diagnosed when polyps were first found at sigmoidoscopy. Radiologically the predominant polyp size was more than 5 mm in only four cases, 2-5 mm in 22 (81%), and less than 2 mm in one. Of the 22 with predominately 2-5 mm polyps, eight had significant numbers of nodules smaller than 2 mm and three had considerable numbers of polyps larger than 5 mm. Eleven (41%) were thought to have fewer than 70 polyps. Pathologically the nodular pattern (less than 2 mm) predominated in 11 (41%) and 14 had polyps of 2-5 mm. More than 100 polyps were present in each case, with fewer than 500 polyps in eight. In the 11 patients thought radiologically to have fewer than 70 polyps, the nodular pattern predominated in nine. In the initial stages of polyp growth, the larger polyps are less numerous, and the background nodular pattern is a useful diagnostic feature of familial polyposis.