Of 65 patients with Barrett's oesophagus, all of whom were white, 29 (44.6%) had colonic tumours; 19 were benign and 10 were malignant. The frequency of colonic cancer was especially high (38%) in patients with Barrett's oesophagus aged 64 years or older. 7 of the 65 patients had had previous colonic resections for cancer and 3 were found prospectively to have malignant tumours. Benign tumours had been removed by colonoscopic polypectomy in 3 of the 19 patients; in 16 patients the tumours were diagnosed prospectively. In comparison with two other high risk groups, malignant tumours were found in 7.7% of 325 symptomatic patients admitted for colonoscopy and in 11.3% of 120 symptom-free patients with occult blood in the stools. The probability of finding this number of malignant tumours in an age-matched random sample of the general population is less than one in a million.