Twenty-nine flat adenomas of the colon from 18 patients were identified by histologic review of 340 surgically or colonoscopically removed adenomas from 210 patients. All lesions had a radial diameter of 1.0 cm or less. Twelve of 29 flat adenomas (41%) contained high-grade epithelial dysplasia, while only five of 127 polypoid tubular adenomas 1.0 cm in diameter or less (4%) contained high-grade epithelial dysplasia. Nine patients had multiple flat adenomas, and two patients had concurrent flat, ulcerated colonic carcinomas without an identifiable polypoid precursor adenoma. Colonoscopically and grossly, the lesions were described as sessile or flat, slightly raised plaques, which might be easily missed on colonoscopic examination. These findings suggest that flat adenomas may be a subtype of colonic adenomas with a propensity for development of high-grade epithelial dysplasia at a small size. These lesions may be precursors of small, flat, ulcerated colonic carcinomas. Heightened colonoscopic surveillance of patients in whom flat adenomas have been identified may be warranted.