From 1964 to 1990, 70 patients with familial adenomatous polyposis were diagnosed at an institution without a polyposis registry. Those with symptoms at diagnosis were older (mean 34 v. 24 years) and more often had large-bowel cancer (7/30 v. 1/30, 23% v. 3%). The introduction of systematic screening significantly increased the number of cases diagnosed annually, from 2,3 to 5 per year, reduced the median age at diagnosis from 29 to 21 years and increased the proportion of cases diagnosed without symptoms from 52% to 90%. A colectomy with an ileorectal anastomosis achieved a low incidence of rectal cancer at 20 years (1/15, 7%) despite imperfect follow-up and annual sigmoidoscopy in only 40%. However, bowel cancer caused at least 35% of all deaths and 62% of deaths due to a known cause. A registry which maintained a screening programme should therefore prevent most large-bowel cancers and improve the life expectancy of patients with familial adenomatous polyposis who are managed at this institution. It might also refine the current method of screening by sigmoidoscopy alone, by facilitating the use of ophthalmoscopy and blood tests for DNA markers.