From 1979 to 1982, 1233 symptom-free subjects at high risk for colon cancer because of family history and/or personal history of bowel neoplasia (cancer or adenomatous polyp) were examined with a guaiac test (Hemoccult II) for occult blood in stools. The test was positive (H+) in 98 subjects (7.9%). Endoscopy was subsequently performed on 86% of the H+ and on 64% of the H- subjects. Of 20 in invasive cancers found, 15 had been H+ [75.0%; 95% confidence interval (CI), 54.3-91.0%]. Of 96 patients with adenoma(s), 23 were H+ (24%; 95% CI, 16.0-33.0%). However, the sensitivity for adenomas was higher in patients with multiple adenomas or with a single adenoma measuring 2 cm or more in its largest diameter (37.5%; 95% CI, 21.8-54.7%). Of 699 subjects free of neoplastic lesions at endoscopy, 47 had been H+, ie, false positive (6.7%; 95% CI, 5.0-8.7%). Adjusting for differential compliance of H+ and H- subjects to endoscopy, a corrected estimate for sensitivity would be 69% for cancer and 19% for adenomas; the corrected estimate for the false-positive rate would be 5%.