The American Cancer Society now advocates screening asymptomatic populations over the age of 40 to 50 years for colonic malignancy. It is possible, however, that fecal occult blood testing and sigmoidoscopy to 60 cm would fail to identify patients with adenomatous polyps located only in the proximal colon. The purpose of this study was to determine how many patients without polyps in the distal 60 cm of the colon would have more proximal lesions. Ninety-eight consecutive patients with positive fecal blood test results or suspicious barium enema results were studied, using the assumption that if they had been seen six months earlier, when asymptomatic, they would have been candidates for screening examination. Forty-one (42%) of 98 patients had adenomatous polyps or cancer, and 15 (37%) of these patients had isolated proximal lesions. The polyp detection rate from 0 to 60 cm was significantly less than the detection rate for a full colonoscopy (27% vs 42%). We conclude that isolated proximal colonic polyps may be common.