This prospective study is designed to determine if experienced sigmoidoscopists can, from gross appearance, differentiate adenomas and hyperplastic polyps. Six hundred eleven polyps discovered at fiberoptic sigmoidoscopy were removed completely or biopsied for histologic examination. The polyp's size, distance from the anal verge, color, and the endoscopist's diagnosis were analyzed. Hyperplastic polyps were significantly smaller, of lighter color, and located closer to the anus than adenomas. The sensitivity of the endoscopic diagnosis for adenomas is 0.80 and the specificity 0.71. We conclude the endoscopic diagnosis of polyps 1.0 cm and smaller is not accurate enough for decision making, and these should be biopsied to guide management. Since 97% of polyps larger than 1.0 cm are adenomas, their removal can be advised without biopsy.