Aberrant crypt foci (ACF) are grossly invisible putative premalignant lesions in the colon. As dysplasia is considered an important precursor of colon carcinoma, we wanted to determine the presence and severity of dysplasia in human ACF. Fifty ACF from 28 patients were embedded in paraffin, cut serially, and stained with hematoxylin and eosin. Multiple slides from each ACF were evaluated for dysplasia according to a defined set of criteria. Of 50 ACF, 3 (6%) contained focal areas with severe dysplasia, ie, carcinoma in situ, 4 (8%) contained focal areas with moderate dysplasia, and 20 (40%) contained focal areas with mild dysplasia. Twenty-three ACF (46%) contained no detectable dysplasia. In 15 of 27 ACF with dysplasia, less than 50% (eg, 4 of 28, 10 of 54, and 10 of 30 sections) of the sections cut and evaluated from each ACF demonstrated dysplasia. The presence of dysplasia in a large proportion of ACF supports the hypothesis that they may be precarcinomatous.