Sixty-eight patients from 18 families have been identified as having familial adenomatous polyposis during the past 30 years in Northern Ireland (population 1.5 million). Six of the 18 probands (33%) had developed colonic carcinoma when first seen at mean age 34 years. Ten of the 44 patients identified by surgical screening (21%) at a significantly lower mean age of 23 years had colonic carcinoma. Surgical management has generally been by subtotal colectomy with ileorectal anastomosis, or by panproctocolectomy and ileostomy.