Small light-red sessile colorectal polyps, 1-5 mm in diameter, are generally believed to be hyperplastic (non-neoplastic), whereas polyps that are dark-red in contrast to the surrounding mucosa are thought to be adenomas. This hypothesis was tested prospectively in a series of 209 polyps removed by one endoscopist in a 6-month period. A 'blinded' pathologist provided the histopathological diagnosis. Twenty-two per cent of polyps described as hyperplastic by the endoscopist were adenomas, whereas 40% of those believed to be adenomas, 1-5 mm in diameter, were hyperplastic. The diagnostic effectivity of the endoscopist increased with increasing size of the polyps. In conclusion, all colorectal polyps must be removed to prevent possible precancerous lesions from being left behind and to define a group of patients with increased risk of new adenomas, in whom follow-up examinations should be considered.