Total colectomy with ileorectal anastomosis (IRA) in familial adenomatous polyposis (FAP) leaves patients at risk for rectal cancer. To assess this risk, the rectal cancer incidence in 297 patients with FAP undergoing IRA since 1951 was determined in the population-based registers of Denmark, Finland and Sweden. At the same time, detailed data on 50 patients with FAP and invasive rectal cancer were obtained from 11 international polyposis registries. The cumulative incidence of rectal cancer was 13.1 per cent at 25 years. The 5-year survival rate of patients with FAP developing rectal cancer was 71 per cent. Combining both studies, the risk of dying from rectal cancer after IRA was 2.0 per cent at 15 years of follow-up. These results justify IRA as primary treatment for most patients; restorative proctocolectomy is preferred for some subgroups. The high all-cause mortality rate observed in this relatively young population necessitates lifelong surveillance of patients with FAP.