Five thousand and twelve asymptomatic patients over the age of 40 years who were on a normal diet were asked by their general practitioners to perform a Hemoccult (Röhn Pharma (Noristan] faecal occult blood test over 3 days. Of the 3 422 patients (68%) who completed the test, 99 (3%) had a positive result. The patients with a positive result then again performed the Hemoccult test, this time over 6 days on a restricted diet. Thirty-two of these patients had a positive result on the second test. Only the 32 patients who had a positive result for the second test were fully investigated (including double-contrast barium enema and colonoscopy); 27 patients (84%) were found to have neoplastic disease. Twelve had invasive carcinoma (9 Dukes' A, 2 Dukes' B and 1 Dukes' C), 6 of these also having 10 adenomas, while the other 15 had 27 adenomas which were identified and removed at colonoscopy. Barium enema missed 3 carcinomas (all Dukes' A) and identified only 16 (43%) of the 37 adenomas. Those patients found to have a negative result for the second Hemoccult test are being followed up.